Continuity mistake: In the first scenes when Weaver first encountered the truck he is pictured in various states of seatbelt wearing. In some scenes it's down and across, in others nothing.
Continuity mistake: When David returns on his way after sleeping, he first wears his glasses in a shot, but when he suddenly brakes after seeing the truck stopped on the side of the highway, his glasses are gone. (01:03:07)
Answer: It's never explained why but, judging from the numerous license plates from other vehicles attached to his truck, the truck driver is a serial killer and was just targeting people at random and decided to make David his next target.
Trucks used to have to be registered in multiple states if they were going to operate in them. It was very common to see semi trucks with a half dozen or so license plates on them, each from a different state.