Question: Why did the truck driver want to kill David Mann?
Answer: According to director Steven Spielberg, the trucker was a serial killer, and each of the license plates were trophies from previous victims.
Answer: David pulled ahead of the truck not once but twice. He got to the gas station first, and got served before the trucker. For any normal person this would not be cause to try to murder someone, however the truck driver is an unhinged psychopath who doesn't need much reason to go into an obsessive rage. Add to that the fact that David is driving a much smaller car, and the fact that they're out in the middle of nowhere with nary a cop around, and the truck driver probably saw David as easy prey.
Answer: "It was very common to see semi trucks with a half dozen or so license plates on them, each from a different state." And that's how the psycho trucker got away with it. He could have those 'trophy' plates on the front of the truck in plain sight, and to anyone who saw it, the truck would look like just another big rig with multiple license plates. Nothing out of the ordinary or suspicious about it.
Question: One of the doctors Phibes blames for his wife's death during an operation is Dr. Hargreaves, whose head is crushed inside a frog mask during a costume ball. But earlier, while introducing himself to Phibes, Dr. Hargreaves says he is a psychiatrist ("headshrinker"). Why would a psychiatrist be present during an operation to save someone's life?
Answer: A psychiatrist is still an M.D., and still went to medical school. Perhaps this one had some knowledge of the case, or was simply on hand to provide assistance.
Answer: To treat anticipated shock.
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.