When Tyrone (the driver) backs into the truck with Frankie the Four Fingers in it, Frankie falls towards the front of the van. Since the truck was rear-ended, he should've fallen towards the back door.
Your Comment here is untrue - If you had ever been in a car crash in which a car smashes into you from the back you, your body is thrown front hence the need to airbags. So this is not a mistake in the film.
Actually, he could have fallen towards the back of the van or the front. If Freddy was in the van at such a position where his balance was weak and could easily be knocked over, then he would have fallen towards the back because the force of teh floor of the van moving forward would have caused his feet to fly forward and the rest of him to fly backward. However, if Freddy had a firm stance or crouching position, his body could have gone towards the front of the van because when the floor moved forward, so would his feet, and the rest of his body would follow. I.E. leaning towards the front or being tensed up.