Originally Posted by Tornreaper
What about timelines that we have already seen? It seems (IMO) that a time traveler doesn't catch up with the timestream unless they either die or change them from time traveling in the first place. That's why Doc turned into FCB. That's why Marty doesn't recognize the timeline changes because although it's part of his timeline he never experienced it. If Doc still traveled though time then he would be with Marty and have no idea that he runs Hill Valley.
So it makes sense. If some changes result in Marty traveling through time then the change will not affect them.
My guess is the second Marty is part of our final timeline and the forth is maybe part of the "Hell Valley" timeline. I'm not so sure about the blue one though.
Doc changes into FCB because Doc has spent over two decades away from his timeline. He's not the same age as FCB n 1986.
Isn't the blue Delorean Marty the one who marries Tiff Tannen? Kind of weird for him to end up with his step sister.
Think of it this way;
-Lone Pines 2015 Biff goes back to 1955. We see him in 1955 during what is now the biffhoric timeline (where it's implied he doesnt live until 2015). In the eastwood timeline he could get another time machine and go back to 1955 again and we have the same scenario.
The reason those 3 marty's show up at that specific time is that it's the earlier they can show up without preventing the critical 1931 trips. All 3 of them were likely that 1986 Marty at some point in their timelines. The timelines skewed at some point