I suspect that they started with a goal similar to: Doc and Marty go to the future. Everything after this point is pretty much secondary to the whole "we're in the future!" thing, which to be honest always worked for me. The 2nd is my favorite because of it.
Since they were determined to put Doc and Marty in the future (because let's be honest, it would have sucked if they never went forward!) the story writing was then constrained by that and probably wasn't allowed to develop naturally. They wrote things in to justify them going to the future instead of writing something that requires them to go to the future.
That's what I think caused it anyways.
Nor
You are a big thinker!
I like big thinkers.
Mismo
Nah I'm just a guy with a computer and with to much time on his hands.