 After Biff gets his cane stuck in the DeLorean after return to 2015,  he starts to thrash around in pain, apparently having a hard time  from the exertion. However, Robert Zemeckis (qv) has stated that  Biff was experiencing the same thing that Marty did at the dance in  the first movie: fading out of history. The idea was that, by going  back in time to give the sports almanac to his younger self, Biff  had somehow altered his own future so that he no longer existed in  the year 2015. Apparently, Zemeckis said, Biffs wife (Martys  mother) had shot and killed Biff sometime after 1985. A small scene  was clipped from the end which made this much clearer as we actually  see Biff vanish.