In this scene from Back to the Future Part II Marty is about to be run over, but Doc saves him.

How did Doc Brown know to drop the rope? (I read a fan hypothesis here.)

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    Looks like just a plot device to facilitate the situation - but one can always argue/speculate that because he was following he could see the situation and plan the rescue. – Allan S. Hansen Dec 18 '15 at 13:24
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    Fan theories are always fun, but they're no match for an actual answer based on evidence. – user7812 Dec 18 '15 at 16:21
  • Suspension of Disbelief. Note, also, that much of the beginning of that clip has Marty perpendicular to the car with nothing for the hoverboard to hover off of. It defies physics, and defies what the hoverboard is even supposed to do. – Johnny Bones Nov 8 '16 at 23:33

The simple answer is that the Doc saw Marty and Biff go into the tunnel, with Marty hanging onto the side of Biff's car. He then flew over the top of the mountain to sit at the other end.

When he didn't see them come out, he rightly surmised that Marty must have let go of Biff's car and would be being chased by Biff back the way he came.

From the film's official novelisation;

So where was Marty?

Doc had kept the DeLorean hovering over the exit to the tunnel for the last minute, expecting Biff’s car to show up at any second. But there was no sign of Biff, and no sign of Marty - no sign of anything, really, except a big old truck carrying fertilizer that had gone into the tunnel beneath him.

Gone into the tunnel?

Oh no, Doc Brown thought, Marty didn’t have to come out this way at all.

He raised the DeLorean quickly. He only hoped he wasn’t too late to rectify his error.

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There is another explanation, which is a dark, uncanon theory:

Marty actually dies and doc finds it out later and goes back in time to rescue him. And perhaps that's not even the first time that happens either! Remember when Biff points a gun to Marty in 1985A? Marty jumps off the building and stands over the flying DeLorean. Maybe he died once. How did Doc Brown know where Marty was anyway? Deus Ex Machina?

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    Any chance of finding corroborating information about this from credible sources? – John Nov 8 '16 at 23:11
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    @John, even if there were no other sources, isn't a pretty "uncanon" theory a nice thing, sitting by your cup of tee? – n611x007 Jan 4 '17 at 20:26
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    @Apollo It surely is. I welcome your answer. I have done similarly numerous times here -- even just a few minutes ago. I hope you enjoy this place as much as I have. – John Jan 4 '17 at 21:00

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