I recently re-watched the Back to the Future movies, mostly so I could write about sleep and how there seems to be time-travel-related jet-lag, but it's also given me an opportunity to write about the alternate timelines in Back to the Future, Part II, a feature that's bothered me for years. The movie doesn't follow its own logic.
The alternate timeline part is explained after Marty McFly and Dr. Brown return to 1985 but find that it's vastly different. Doc says that "Prior to this point in time, somewhere in the past, the timeline skewed into this tangent, creating an alternate 1985." He even draws a diagram:
This 1985 is alternate because Biff from 2015 went back to 1955 and changed the course of future history by giving his past self the sports almanac. But since 2015 Biff changed the course of future history when he's in 1955, when he returns to 2015, it should be the 2015 of that now-alternate timeline - the same timeline that Doc and Marty experience in 1985. Instead, the movie shows 2015 Biff returning to the same 2015:
Like the newspapers change after key events are prevented later in the movie, 2015 should change around Doc and Marty from the "original" 2015 to the alternate 2015. I'm not sure what would happen to Biff though.
It might be that 2015 Biff can return to the original 2015 because that skewing of the timeline would change his own future so much. So he's returned to his own time, which then vanishes as the now-altered timeline takes prominence. That's the only explanation I can think of, but it still seems a contrivance just so that the Delorean can be returned to Doc and Marty.
