If you've ever been on a farm you will know that animals have a mind their own. Mules, in particular, are known for their stubbornness, they either want to go or they don't. Stormy always did.
I think that Stormy, perhaps, was a dog in the wrong body. As dogs like to ride in a vehicle with their tongue lolling out, Stormy liked to ride in the back of the truck, looking over the roof.
He loved it enough that he climbed aboard by himself. His partner, had a small pickup truck with a complete box minus the tailgate. There were no stock racks or loading platform. Stormy got his front legs up and the rest followed making him a self loading mule.
It was not unusual to see him at Gymkhanas and other events where he came to socialize. We can just be thankful he always found the right ride home.