I think my #1 is medical event. I have (variously) a 3500 dually truck, a 37' camper, a 30-something foot 6 horse trailer and a little 3 horse bumper pull which I've got out now. I use the (semi) truck pumps at gas stations because my diesel truck takes DEF (diesel exhaust fluid) and they have it on the pumps. So I'm hauling pretty big stuff and around even bigger stuff frequently. The trucks are just so big. You'd never want to load or drive the things if you didn't have to.
The only real opportunity for foul play would be at the loading site and if something happened to him there, it would make no sense to even load or drive the truck. Otherwise he would have had to have a second person in the truck who quietly overpowered him to leave the truck parked in a lane and make off with him. It just doesn't track to me as someone who hauls animals all over the US. The main thing you want to do when you're hauling animals is get there. If he wanted to do some sort of other activity, he could have done that before he picked up the load (or skipped it altogether).