Who caused the bus crash?

According to this article, the doctor was the one who said that Myers crashed the bus (I couldn't remember either). So it's up to you to decide if the doctor at least assisted him or if Myers did it all by himself to the (fairly short-lived) surprise and delight of the doctor. Both scenarios are believable. If you're not convinced that this was just a happy/fatal coincidence for the doctor, you could hypothesise that he was going to try and spring Myers at another point but was not expecting Myers to cause the bus crash of his own volition.

Michael’s first kills come when he’s being transported by bus to a new prison. The bus driver doesn’t die onscreen, and his body isn’t showcased like so many of the other corpses. But we do learn from Dr. Sartain that Michael overpowered the bus guard and the bus driver, causing the fatal crash that set him free. It’s safe to assume the driver either died in the crash or was murdered shortly thereafter. The same goes for the bus guard, who at least tries to warn Michael's next victims before he dies.

While I don't have much more "hard" evidence than anyone else for either theory of who caused the crash, I would make a strong case for the doctor doing it, or at least being ultimately responsible for it, based on what the film tells us about him.

It was the sole reason why the doctor was on the bus in the first place afterall. It apparently wasn't a common thing that he would accompany his patient as the guards protest it at first. And while you could argue that he was primarily concerned about Michael, we have to figure in that this was basically his last chance to make any progress with him.

You have to see where the doctor stands at the beginning of the film. He has spent his entire carreer trying to understand Michael Myers, reading everything about him and trying to talk with him. But now he has not only reached a dead end (he's reached that long ago), Michael is about to get locked up in a deep dark hole forever, without Dr. Sartain ever getting a chance to figure him out. It was his life's work that was at stake here.

Dana: We were hoping to have this opportunity before he's transferred to the new facility. Glass Hill is far less accommodating.
Sartain: Glass Hill is the pit of hell. For years, he's been kept here to be studied. I suppose the state has lost interest in discovering anything further...Michael has been my life's obsession. I've examined every single case file written on him. I was a student of Dr. Loomis before he passed away. And then I lobbied the University of Illinois to be assigned to Michael myself.

Afterall, it is Michael Myers's complete and utter emptiness and inexplicability that makes for a lot of his fascination and this dilemma is what Dr. Sartain got trapped in during his decades of work with Michael. He wants to understand him and can't accept that he cannot be understood, something his mentor Dr. Loomis had already realized long ago. In fact Dr. Sartain as someone going as far as killing people in order to understand is somewhat of an antithesis to Dr. Loomis, who was the other extreme of a psychiatrist not only abandoning any chance for understanding or even cure, but actively advocating for eliminating Michael for being not human.

Dr. Loomis was the only one to see him in the wild. And he concluded he was nothing more than pure evil.

So he didn't just get obsessed with Michael once he saw him escape and decided to observe him in the wild. That obsession grew for a long time and he needed to set him free as that was the only chance for him to ever make real progress, not by taking him out of jurisdiction but by actually seeing him "do his thing". Even if they catch him again in the process, this was the best and last chance for learning something the doctor had for decades and he needed to grasp it.

Now it's still unclear if the doctor had a direct hand in killing anyone on the bus or if he just loosened Michael's handcuffs and let nature take its course, but from his character and the situation he was in, I very much think freeing him was the very reason for the doctor to be on the bus at all. And the latter case would still be in line with your reasoning that noone else mentioned the doctor's involvement and the doctor hadn't killed before Officer Hawkins.