Ghouls. Agreed, when you look at the CCTV there are no flames or explosion and in the zoomed in part you can see the bus lurch, stop and pale grey smoke starting to billow over the bus, which indicates to me extreme heat has generated from under the chassis area fanned by the forward movement of the bus, when stopped erupted into flame and fuelled by the insulation, wall and ceiling panels, seats, quickly got a hold and spread. You can see other ghoul-footage of the bus burning and the 'seat' of the fire is clearly discernible.
The Road Transport and Fire Investigators will prove the origins of the fire.