> one is left wondering how the vehicle managed to catch fire in the first place.

Any heavy vehicle with _friction_ brakes can generate enough heat to create fire, it turns out spinning metal is always a stored energy hazard regardless of how the motive power was originally supplied.

Plus upholstery is pretty flammable. They lace it with chemicals to try to retard that tendency but after a certain point they're going to light up and fuel the fire.