Question: We see Biff groan in pain as he returns to 2015 with the stolen DeLorean. In a deleted scene we actually see Biff fade away as he is erased from existence by his actions. I assume this is because an alternate Biff now exists (or could have been killed) and so he is erased from time. If this happens to Biff, why doesn't this happen to Marty, Doc, and Jennifer? They too are replaced by alternate versions of themselves when they return and even before they return to 1985a. 1985a Biff even says in the film that Marty was in school in Switzerland, and Doc has been committed, so they too should fade away as Biff did, shouldn't they? I would say "time" was giving them time enough to correct things like in the 1st movie, but Biff faded away almost a soon as he arrived back in 2015a.
Captain Defenestrator
9th May 2009
Back to the Future Part II (1989)
Chosen answer: Biff faded away because he completely changed his own past. When Marty, Doc, and Jennifer return to 1985, they're returning to the alternate timeline that Biff created. They still exist and remember their own history because without it, Biff's timeline couldn't exist and a paradox would be created.
Answer: The comics answer this question. In 1986 in the alternate timeline, Biff forced the committed Doc Brown to send him forward in time to get more sports results, but Doc tricked him and sent him back to 1884 instead, where he was shot by his own great-grandfather Buford. When he reappeared in his current time (that's how the time machine worked in this timeline), he was dead. Old Biff faded from existence because Biff never grew up to become Old Biff (because he was dead), so Old Biff no longer existed.
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Answer: I read somewhere the reason Biff faded away was Lorraine shot him in the alternate timeline for murdering her husband, George.
I hadn't heard that one, could be in a novelization or something. This is entirely my own speculation with nothing official to support it; however, it could be that because Doc and Marty are trying to correct the timeline, he and his siblings' removal from history happens gradually, as they get closer or further from the act of their trying to set things right. Biff doesn't know or care and is actively trying to change history, so the moment he returns to 2015, he vanishes all at once.
Captain Defenestrator
If you have the DVD or Blu-ray, watch the deleted scene of Biff vanishing and turn on the commentary. It will confirm that Biff was murdered, which is why he was erased from existence.