Preobably an engine fire, although I've seen quite a few engine fires before and rarely one that big (usually they stay fairly well contained in the engine compartment, and that goes for any vehicle). Could be the speed (and therefore airflow) that did it, allowing it to get to a high enough temperature to really spread. It would have been running flat out on the bypass before this. The fact it was at speed might also have prevented the driver or passengers noticing anything was wrong until it had really taken hold, as smoke and flames would have been drawn backwards away from the bus.