Continuity mistake: Throughout the movie, there is a family picture that disappears and reappears on the driver's side sun visor.
Continuity mistake: While David is sitting at the table in Chuck's Cafe, the ashtray is unused, but moves around the table several times. Cutlery changes position too, in two different shots.
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.