Why is Doc Brown in a hurry in the end of Back to the Future part 1?

I hope I don't spoil the party with an out-of-universe explanation, but I thought that this was really interesting when I found it out: It was just supposed to be funny. The sequels weren't written yet.

In the commentary on the DVD, Robert Zemeckis and Bob Gale explain the scene, that it was all just there for fun, including the "To be continued…". At the time, they weren't planning on making a sequel, and in their own words, were just hoping that the first movie would break even at the box office. This scene was just a sort of joke.

Afterward, when the film was such a success that they were asked to write a script for a sequel, they regretted writing this scene the way that they did, because Doc and Marty brought Jennifer along, which limited their options in writing the next script. If it has just been Doc and Marty, they could have made the next movie about anything they wanted, the two guys on any crazy adventure in time. With Jennifer around (and what Doc said), the second movie now had to be about their kids. They had "painted themselves into a corner" in a way, but still managed to make an excellent story out of it!

Whenever Doc Brown reaches an epiphany, he immediately sets to work to fulfill the context of said event. As an eccentric scientist, he has become accustomed to working in a high strung and over-driven manner, as he is always eager to have his projects come to fruition. With regards to what he concluded about Marty's children, he was alarmed by their unfortunate delinquency. Therefore, his perspective was that his epiphany needed his fullest attention, despite time being irrelevant in relation to the urgency.

My theory is Doc was lying. He tells Jennifer and Marty that a cascade of bad events started with their kids at this one event in the future. Truth is it all started within an hour of that moment when getting warned by Doc. Marty gets into a car accident with his truck and injures his hand that ends his ability to play guitar. He may have not became famous but that incident in Marty's teenaged life made him pretty much give up on his dreams and to become a better man. Marty up to that point had about 17 years with a weak timid father and borderline alcoholic mother and after all that time traveling he spent maybe 30 minutes with the confident and successful family before he takes that drive. So Marty still has in the back of his head a fear of turning into his parents and after the accident accepts that his dreams are crushed and he just settles for things as they are. When Doc goes to the future he sees what Marty has become and decides that he needs to expose both Marty and Jennifer to this so they can change his path for the better. It wouldn’t be enough just to stop the accident but to completely help Marty evolve.