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: 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.
Continuity mistake: At the end of the film, David Man comes to a dead end, so he spins the car around and waits for the truck to appear. When the truck appears from over the hill, take a look at the very next shot of the truck from the front, and you will notice that the large exhaust on the side of the truck has moved from left to right and back to left again. (01:21:00)
Continuity mistake: When David Mann is at the cafe, the waitress brings over his sandwich and water. He immediately drinks all of the water, yet when the shot changes to above the table, of him looking at his sandwich, the glass of water is completely full again.
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.
Continuity mistake: Towards the end of the movie where David begins to take advantage over the truck driver, and he turns on right towards the climb also breaking the cartels and fences, the weather changes in two shots: it's cloudy before and sunny immediately after.
Continuity mistake: When David returns on his way after sleeping, he first wears his glasses in a shot, but when he suddenly brakes after seeing the truck stopped on the side of the highway, his glasses are gone. (01:03:07)
Continuity mistake: After the truck tried to make David's car hit by the train dabbing it from behind, the car has a dent on the right rear light, but in a following scene, the dent has vanished. (00:53:09 - 00:54:30)
Continuity mistake: When David has the accident and crashes his car against the fences, he steps out of it and closes the door, but when he comes back to go on his own way, the car door is inexplicably open. (00:26:49 - 00:42:52)
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.
Continuity mistake: When David Mann crashes his car against the truck near the end of the movie, the car's frontal bodywork inexplicably returns as brand new in one shot and back smashed again.
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.