Trivia: Steven Spielberg "interviewed" over 100 trucks before finding the forboding, evil looking one for the movie.
Trivia: At the end of the film when the truck goes off the cliff, we hear an odd groaning sound. This is a dinosaur growl from an old movie Spielberg likes. Interestingly, this same sound is also used at the end of Jaws as the shark sinks to the bottom of the ocean. Spielberg did this to show that Jaws and Duel would always be closely connected.
Trivia: This film garnered so much critical praise that it was theatrically released in quite a few countries, though it was originally a made-for-TV-movie for NBC in 1972.
Trivia: The scene where the truck goes over the cliff had to be perfect the first time - the production team could not afford to purchase other trucks due to the extremely low budget.
Trivia: This was the full-length feature directorial debut of Steven Spielberg.
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.