Originally Posted by Jennifer
To me, I've always thought of as a ripple effect in two parts. Biff comes back to 1985 and he doesn't disappear right away because of the time ripple effect as mentioned on the Back to the Future DVD. The future immediately changes around Doc, Marty and Jennifer, but they don't notice it since they're outside the former McFly residence at that point. The way I see it, the reason Doc, Marty, and Jennifer don't change even though their surroundings do is because time reacts differently to someone who has traveled outside their own timeline. The ripple effect takes longer to effect Marty, Doc, and Jennifer because they're displaced from their original timeline.
I know that was just a typo and you meant biff comes back to 2015
and yes it could be that it wouldnt make sense for doc and marty to be in 2015 and still remember the alternate timeline since they likely dont time travel in the alternate timeline. Or simply they got out before the ripple effect took place. This is the only instance in the trilogy which takes place immediately after the timeline changes; in part I marty leaves about a half hour after the last ripple (getting lorraine and george back together and george punching out biff) and doesnt get to see the effects til morning. When they go back to 1955 the second time, it's a while before they go back to the present and the only change in 1885 is eastwood ravine. In part II we do see the matchbook, marty's newspaper, and doc newspapers all change at different times despite one event (burning the almanac) causing all the effects; they do all change seconds from each other but still not at the same time meaning the ripple effect is not instant.