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.
Continuity mistake: On the truck cab's rear window, there is some sort of air conditioning unit installed. In some shots, such as the gas station scene where David Mann is on the telephone, or the railroad crossing scene, the air conditioning unit has a support structure to hold it in place. In the rest of the film however, there is not any support structure to be seen.
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.
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.
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.
Continuity mistake: During the entire movie each time the camera pans on the car's speedometer, the odometer clearly show completely different numbers in the cars mileage which are thousands of miles apart.
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: As David Mann starts fuming at the guy who he thinks is the trucker, the first shot shows him taking a sip of his beer; it is almost empty. As he says "What are you talkin' about?" its almost up to the brim.
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)
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)
Continuity mistake: There are moments in the film, notably during the railroad and pushing sequence where the car's appearance - such as being dirty or clean, changes somewhat; the truck changes during that sequence as well. (The film was first broadcast on TV, then later released to theatres, and the push sequence was specifically filmed for the theatrical release. An identical '71 Plymouth Valiant was used, but a very different truck.)
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?
Continuity mistake: When David Mann is at the service station, window cleaner is sprayed on his windshield from the driver's side (camera shot from inside the car). Within a milli-second of the spray stopping, the service attendant is seen on the passenger side of the car. We are only shown one attendant at this station.
Continuity mistake: Near the start of the film, David stops at the gas station. We see him get out of the car and he asks the attendant if he has got change for the telephone. We can see David take out his wallet and pull out a note with the other hand, then puts his wallet away. Then the camera angle changes and his wallet is back in his hand again and he puts it away again. Also you never see the attendant take the note out of Davids hand. It just disappears. (00:10:45)
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.
Continuity mistake: When David crashes his car into the fence at Chuck's cafe, it shows that it was the passenger side that crashed into the fence. The driver side is undamaged. However when David exits his car to help the bus driver, there is significant damage to the driver's side of his car. (00:26:40 - 00:44:25)
Continuity mistake: When David Mann is flagged down by the bus driver, the window of the rear driver's side door is up. After Mann's 'just bounce it loose' speech, the window is down and remains down for the rest of the scene.
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.