Revealing mistake: At the reptile farm, the phone booth Mann uses to try and call for help has no phone lines attached to it, above or below ground (you can see that there are no underground wires when the truck mows the booth down). It's definitely not a wireless/radio telephone, either. There should be lines running to it, but there are none. (00:57:00)
Revealing mistake: Just before the truck slides off the cliff edge, you can see that the truck's cab door is open where the stunt driver jumped out.
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.