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.
It may be a while before we are told what really happened.
One thing for sure and that is I no longer have a desire to own a bus as an RV.......