Visible crew/equipment: When Dennis Weaver is in the cafe a shadow of a cameraman is cast across the table he is sitting at. The shot then changes to a wider angle to reveal that there isn't anyone near him who could have cast that shadow.
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.
Visible crew/equipment: Just as Dennis Weaver goes back into the Diner after splashing his face with water in the washroom, you can see the shadow of the camera man, complete with camera on his shoulder, silhouetted on the back of his shirt and on the wall.
Visible crew/equipment: When David Mann stops off at the reptile lady's place, he enters a phone booth to call the police. When he's inside, you can see a perfect reflection of Spielberg. Look how young he looks.
Visible crew/equipment: When Mann's bumper gets caught under the school bus and he's trying to free it, a large lighting panel is reflected in his car door every time he opens it. (00:47:00)
Visible crew/equipment: Just after the truck has run down the phone booth at the Reptile lady's place, it starts smashing into all the glass reptile cages nearby. If you look closely at the reptile cage which David falls over, you can see the film crew in the reflection. (00:58:05)
Visible crew/equipment: During the scene in the gas station with snakes, as Dennis Weaver is wiping the tarantula from his leg, look at the shadow of the old woman behind him. There appears to be a moving crew member shadow next to hers. It cannot be a tank as she wasn't standing next to one either in the shot before or after. (00:55:50)
Visible crew/equipment: The shadow of the camera can be seen on the left side of the screen, after David takes his seat at Chuck's Cafe.
Visible crew/equipment: At the gas station, just before the water is sprayed on the windscreen and the attendant first appears, if you look closely at the top right hand corner of the screen you can see a reflection (albeit for a split second) of a hand giving the cue for the action to begin. Was this Steven Spielberg's hand?
Visible crew/equipment: Near the end of the film, while Dennis Weaver is finally starting to get a good lead on the truck, the top of director Steven Spielberg's head is visible in the reflection of the back seat in the rear-view mirror.
Visible crew/equipment: Visible camera on the right side (near the tail lamp) of Mann's Plymouth (before the Chuck's Cafe scenes). (00:22:20)
Visible crew/equipment: When Mann's car spins out on the gravel and comes to rest facing the truck, you can briefly glimpse the cameraman (purported to be Steven Spielberg himself) crouching in the back seat. (01:04:25)
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.