Originally Posted by Michael J Fox is Canadian
Prior to mr. fusion, the delorean needed plutonium for each time travel. It may be possible that with mr fusion, the delorean can time travel multimate times for each time travel. I'd assume that it either had enough to time travel already or perhaps Biff new what mr. fusion was and how to use it. I dont think it is labelled that the delorean must go 88 mph but maybe biff just figured it has to go fast to time travel and eventually got it to 88 mph.
I'm not sure if old biff is smart enough to figure this out but if he did not return the delorean to doc and marty, it would cause a paradox or at least prevent him to get rich; Doc and Marty go to to get Jennifer from her future home and go to the future in the first place to due to their future children. If Marty and Jennifer got stranded in the future, they wouldnt have children there and thus doc would never bring them there in the first place for biff to steal the time machine.
Another potential is that Biff started having his 'symptoms' that we see while in 1955 and that is why he goes back to the future.
Biff specifically remarks to himself "doc brown invented a time machine" and pieces together that one of the 2 "Marty Jrs" he sees is Marty Sr. from the past so he figures when they find out the timeline has changed, they'd come asking him about the almanac.
Now one thing I've wondered; Biff sees the delorean fly and 'disappear' in 1985, the next day he sees marty (after returning from 1885), why wouldnt he ask him what on earth happened? Didn't they wonder where marty went to without his truck? Not sure how those 2 timelines would play out in their eyes as Marty leaves 1985 around 11 am on the 26th, returns to the alternate timeline at 9 AM on the 26th, leaves around 2 AM on the 27th and returns at 11 AM on the 27th.
I thought Mr. Fussion was essentially a car battery? Would it have enough juice for the required 1.21 gigawatts. By the way, I thought it interesting how the time machine still supposedly ran on gas in part 3, even though Doc says "I need fuel" when he puts trash in it at the end of part 1.
It would create a paradox if Marty and Doc were stuck in 2015, though it's hard to say if Biff would know that. Does this mean the Alamac would have disappeared back in 1955 had Doc and Marty been stranded in 2015?