I'm fond of the Back to the Future theory that Doc knew Marty died when Biff was about to run him down in his car at the end of the tunnel in the second movie. Doc drops the flag rope at the last possible second, but there's no way he could've known to do that unless Marty got hit and died the first time which prompted Doc to use the Delorean to jump back to before that happened in order to use that knowledge to save him.