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!