Probably some kind of traumatic experience as a puppy, I would guess.
All our dogs have a thing against UPS trucks, although they just run out and bark at them. Our local UPS guy carries dog biscuits with him, and tosses them biscuits when he leaves a package on the porch. They growl and bark at the truck, but they love the biscuits.
The idiot boxer mix I have now jumps in their truck and wants to go for a ride.
Had the same issue with the Census takers and the cop that came out when joggers claimed he was a vicious dog threatening them.