Smoke from Tow Vehicle

We stopped at a pull out in the lower Tetons last week to take a rest and a cowboy came into the pullout to settle his horses down that were kicking inside his trailer. His truck was a new Chevy and his rear brakes were smoking so bad one of his ranch hands was watching the wheel wells to make sure there wasn't a fire. None of the smoke was coming from between the bed and cab though, all of it was billowing out from the wheel wells.

Possible your transmission got hot and vented fluid? I believe autos have a short vent on top which could then drip down onto things to burn off?