
Visible crew/equipment: When David's car gets stuck on the bus bumper, the next shot is of the bus' black bumper and the side of the car. At the top of the black bumper there are two people seen in the reflection. The person on the left turns and takes a step to the right.






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.