How did Detective Spooner get into the car accident that injured him?

I just stumbled across this question and thought I'd add a few valid reasons as to why the truck may not have been auto-driving. [NOTE: I haven't watched this in a while so the wording of my quotes might be slightly off.]

Technology takes time to spread

As far as I can remember, it's never stated when Spooner's accident took place, but given the amount of surgery and physiotherapy he would have had to undergo for his injuries, it was probably a few years before the story takes place. In 2035, auto-drive is certainly commonplace, but it isn't mandatory or universal - it would logically have been even less commonplace when the accident occurred.

Furthermore, it's possible that the accident itself, and others like it, accelerated the adoption of auto-driving, leading to the apparent ubiquity seen in the film. A truck driver killing a 12-year-old girl after falling asleep at the wheel, when his truck could (and maybe should) have had auto-drive equipped? People would have been outraged.

Technology is expensive

A modern electric car, even with thousands of pounds/dollars of government subsidies, is still a lot more expensive than a comparable petrol-powered car. I dread to think how expensive a self-driving truck would be in comparison to a modern manual truck. It's entirely possible that the truck company kept using outdated manual trucks in order to save money. If I had a penny for every time a real-life company's penny-pinching has ended in disaster, I'd be a rich man.

Technology is unreliable

Granted, humans make mistakes, but so do machines and computers. They break down. They malfunction. They do things we don't expect them to (like start a Zeroth-Law Rebellion, for instance). And so many people, Spooner included, trust the judgement of a human over the judgement of a machine.

SPOONER: 11% is more than enough. A human woulda known that, but a robot... there's nothing in here. [points to his chest] It's just lights and clockwork.

A modern example would be satellite navigation systems, which have been known to lead truck drivers down impossibly narrow paths, or into the middle of fields, or the ocean. So either the company owner didn't trust the auto-driving system enough to install it, or the driver didn't trust the auto-driving system enough to use it.

Technology is confusing

These new-fangled whatchamacallits can be pretty baffling to some people, especially if they haven't been taught how to use it properly. There's a story about an elderly woman who put her campervan on cruise control and then went into the back to make a cup of tea, thinking the cruise control would steer for her as well. It didn't. The campervan crashed.

So it's possible that the driver didn't know how the auto-driving worked, and figured he could take a quick nap and the computer would take over in the meantime. Except it didn't.

An interesting parallel

I want to point out something that I myself didn't realize until I started typing out this answer:

SPOONER: The truck driver... his name was Harold Lloyd. Like the film star, but no relation.

Harold Lloyd (the actor) was a star of black-and-white silent comedies, which were rendered obsolete when talkies and Technicolour arrived, thus ending Lloyd's career. It's possible that the film-makers used the name "Harold Lloyd" as a deliberate allusion to the idea of people being made obsolete by technology, a theme that's also touched upon in Spooner's first conversation with Robertson (mentioned in @sampaco's answer).

With that interpretation in mind, you can imagine that Harold took one look at a self-driving truck and either thought, "That things's going to take my job, I don't like it", or "That thing's going to drive me into a hedge; I don't trust it". Either way, I don't think he'd be setting foot in one, and that sadly led to his demise.

Regarding your points

I respectfully disagree with some of your interpretations.

Dr Calvin expresses disbelief that Spooner is manually driving the vehicle

Because he's going at about 150mph at the time. And talking on the phone. It really is safer for him to leave that to the auto-driving system. Calvin doesn't seem to have a problem with him manually riding a motorbike on two other occasions in the movie, because those times he's going at more manageable speeds.

Later in the movie, he is berated by his superior when they think he caused an accident in the tunnel

The important bit here is "they think he caused an accident in the tunnel". Borgen isn't berating him for switching to manual, he's berating him for switching to manual for the sole apparent purpose of running two USR trucks off the road. If there was an obvious reason for him to have switched to manual, I don't think Borgen would have had a problem with it.

In the movie it is generally accepted by society that the automation is bullet-proof

No. It is generally accepted by society that the Three Laws are bullet-proof; specifically, that robots cannot commit crimes, ever.

CALVIN: Face it, a robot could no more commit murder than a human could... walk on water.

But a piece of automated software consists of much, much more than just the Three Laws. There are hundreds, even thousands of things that could (and probably do) go wrong. I don't think we can extrapolate that 2035 society, as depicted in the film, blindly believes that all automated systems cannot possibly fail, ever.

In conclusion:

1. While the film doesn't seem to contain a definite explanation for why the accident happened, there are enough possible (or even likely) explanations that I wouldn't call it a plothole.
2. I spent way too long writing this answer.

I've decided to revisit and revise my answer for two reasons: you point out that your question involves whether it is a plothole that the semi truck driver would be driving manually or not, and I've recently rewatched the movie with this in mind. So here goes.

There are several assumptions being made which I believe to be false.

• Assumption #1: "pretty much no one" drives manually, at least not at high speeds
• Assumption #2: a commercial semi driver could not be able to disengage auto-drive
• Assumption #3: even if he could disengage it, a commercial semi driver would not disengage auto-drive
• Assumption #4: Spooner's distrust of technology came about after his accident.

Let's attack these one at a time.

False Assumption: pretty much no one drives manually

We simply don't know this for a fact. You are right that it is established that driving manually isn't considered normal at this point at least on the freeway at high speeds. Yet there are several facts to indicate that it is still done and perfectly possible at least by more than just Detective Spooner.

1. Spooner's car HAS manual override - if no one drives manually then why is the override even there?
2. Spooner's car has navigation settings - when he his visiting USR the first time the car tells him to take the next right as he is driving manually at the time. Again, why have these features if no one drives manually?
3. VIKI quotes statistics about traffic fatalities being down 9% - this tells us that traffic fatalities are still a common occurrence which could be very likely due to there being people out there driving manually.
4. We don't know how far removed society is from all manual driving - the movie is set in 2035 and was made in 2004 when Will Smith was 36. If we assume Spooner to be the same age then that means he was born in 1999 and learned to drive at the age of 16 (legal age in the US) in 2015. Society is not many years removed from all manual driving and there are likely people who still prefer it.

We don't know how traffic technology works in this world exactly. It would be highly plausible to assume that it is more normal for automatic driving to be the preferred method at high speeds, yet manual driving be preferred on local roads. Again we simply don't know but there are plenty of plausible explanations.

False Assumption: a commercial semi driver would not or could not drive manually

I can definitely see how it would make sense to think that in a world with manual driving that this would become part of policy or even law. However there is nothing established in the movie to give us more information. We have to go off what we know and what we know is that it is possible to drive manually.

Even if it was against policy or even law for a semi driver to drive manually, since the possibility exists and we know that the semi driver caused the crash by falling asleep at the wheel then its perfectly logical to conclude that the driver was breaking said policy/law.

False Assumption: Spooner's distrust of robots only comes after his accident

This is more of a side point although I think its important because it is not only Spooner that dislikes robots even though this isn't established strongly in the movie. There are several clues.

Spooner is an old fashioned guy

1. He owns a JVC CD player which Calvin can't control using voice commands
2. He wears "2004 vintage" Converse All-Stars
3. He drives manually on a regular basis, not just when robots attack his car. We see his doing so at least 3 times in the movie.

Robot prejudice is a thing

When Spooner visits NSR after Dr Lanning's death and is interviewing Roberston he gives shows his dislike and cynicism towards robots taking over blue collar jobs.

Robertson's response seems to indicate, at least in my opinion, that he has heard all these arguments before.

I suppose your father lost his job to a robot. Or maybe you would have simply banned the Internet to keep the libraries open. Prejudice never shows much reason. I suspect you simply don't like their kind.

The point being that there are plenty of reasons for many people in this world to hate robots and Spooner's attitude seems to be more deeply seeded than to be attribute to one event.

Spooner seems to have a history of distrusting robots

His superior berates him about chasing down the robot with the purse on his way into work on what appears to be his first day back after the accident. The other detectives in the precinct are also teasing him. His superior tells him "this better be the last time".

The later when Bergin and Spooner are at the bar after Robertson issues them the gag order, Bergin tells Spooner he was the right man for the job.

Face it, with all the rocks you've been looking under to find a bad robot, what are the odds you'd be the guy to find one?

All this seems to indicate that Spooner has a history of distrusting robots that runs deeper and precedes his recent accident.

The reason I bring this all up is to make the point that there are likely other people out there who, like Spooner, prefer the old fashioned way and might opt to drive manually at times.

Conclusion

There is not enough information given for us to be able to say this is a plothole.

Wikipedia's definition of a plothole seems appropriate here:

In fiction, a plot hole, plothole or plot error is a gap or inconsistency in a storyline that goes against the flow of logic established by the story's plot. Such inconsistencies include such things as illogical or impossible events, and statements or events that contradict earlier events in the storyline.

I can see how you would think this might be a plothole. However I think there is not enough established logic in the story for us to be able to say that a truck driver would or could not drive manually.