According to insurance sources a Tire Fire like the one on this carhauler occured due to heat buildup in the the brakes or the axle, most likely malfunctioning, when the car slowed down or stopped. The heat buildup around the tire was great enough to get this carhauler nightmare blazing. Tire pressure can also be a factor in these sort of carhauling accidents.
Photo courtesy of http://twincitiesfirewire.com/2011/06/28/lake-johanna-firefighters-battle-intense-car-hauler-fire/