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.
In very slow moving, stop and go traffic, if it were brake related, the heat would have to be very intense to actually ignite, and, if it was brakes, it would seem to me that would have taken time to produce those type of high temperatures to actually ignite.
In this scenario, slow, stop and go traffic, it would just seem the the driver and passengers would have smelled it for a period of time prior to the ignition.
This was probably one of the first questions the investigators asked the survivors.