Later footage indicates the fuel tank may have exploded but by then the fire was raging.
If there was a brake fire, it may have been caused by low air pressure which would apply the brakes with excess heat. Detroit engines are known to be oil leakers without adequate maintenance. That would increase the possibility of a fire.
I was reading other articles on this, and the responding LEOs said it was the worst they'd ever seen, absolutely horrific. Apparently, just prior to the event, one or more motorist were trying to get the drivers attention that the vehicle was smoking. They said at least 24 of the elderly were burned to alive, and some were decapitate. Probably from explosions.