Possibly. The vehicle was in very slow moving traffic. My gut feeling it wasn't brake. I say this because if it were, it would take time, and the heat would have been intense, and the driver would have most likely smelled it long before it actually ignited, since it was in slow moving traffic. Just speculating here.
The engine is in the rear of the bus from the pictures posted. Doubtful that the driver would know anything with the engine 30 something feet behind him. Just speculating as we do not know for sure yet what happened.