 PLOT: At the end of the movie, the hovering Delorean is struck by  lighting thus sending the required amount of electricity into the  flex capacitor to start the time travel process. Part of the  requirement for the Delorean to travel in time is that it goes at  least 88 mph. The Delorean was just hovering in the sky and not  going 88 mph when it was struck by lighting. So being struck by  lightning alone would not cause time travel to occur. Also if  lightning alone could cause the time circuits to operate, Marty  would not have needed to race down the street to hook the cable to  travel from 1955 to 1985. This is also proved when they travel to  2015 and you can see the speedometer rise to 88 mph before time  travel takes place.