David Mann: I'd like to report a truck driver who's been endangering my life.
David Mann: That truck driver's crazy, he's been trying to kill me, I mean it.
Bus Driver: Well, mister, if I was to vote on who's crazy around here, it'd be you.
David Mann: Come on you miserable fat-head, get that fat-ass truck outta my way.
David Mann: Well it's about time, Charlie.
David Mann: Come on, car! come on, LET'S GO.
David Mann: Fill it with Ethel.
Gas Station Attendant: As long as Ethel doesn't mind.
David Mann: You can't beat me on the grade. You can't beat me on the grade.
David Mann: Where's the summit? Please... please... come ooooon.
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.