Continuity mistake: When Rusty Nail has the car pinned up against the tree, his truck breaks the windows. The driver's side window is completely broken in one shot, however when Rusty leaves, the window is shattered but largely intact.
Continuity mistake: During the cornfield scene, there is at least one point where the spotlight on the truck is off, but then on again.
Answer: Because Rusty Nail was a psychopath. That and he must have felt physically threatened. And the innocent guy was mouthing off. Hence, the jaw removal.
ChristmasJonesfan