Yeah, so? I thought that was because Biff was going to ram his car into the truck, and crush marty's legs. I would move to the side if I were to get my legs crushed (the same thing happened to me with a radio flyer wagon and a honda, and that hurt like heck!).
Oh, why does Biff get to drive like a maniac throughout the town square, almost hit two people at the courthouse, destroy hedges, and attempt to kill Marty? All with a bunch of teens watching at the local diner? NOBODY would think to report the police?