Question: I never really understood at the end of this film why exactly Doc Brown goes back to 1885. Doesn't the DeLorean have to be going 88 mph for it to activate the system? The Doc was just hovering over Marty, so why did the time circuits automatically kick in?
XIII
10th Dec 2005
Back to the Future Part II (1989)
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Answer: If you look carefully, Doc accidentally knocks the time circuits on when trying to keep the DeLorean stable. When Doc and Marty were leaving the alternate 1985, we see that the time circuits were failing/faulty. So when the lightning struck the DeLorean, the time circuits were already on and with the equipment faulty, Doc was sent back in time. The DeLorean flips (possibly due to the flight circuits being hit by lightning) and the car accelerates to 88 MPH.
XIII