Having worked at Greyhound Corporate IT, I supported the BOSS platform, which housed fleet maint records. My guess based on the pics on the Birmingham news site, is, the fire may have started from an overheated tire. Either not properly inflated or maybe having picked up a nail in travel. Unless the bus is equipped with TPMS (tire pressure monitoring system) the driver may not have known he was driving with a flat. That bus was a MCI DL102, which didn't have standard TPMS. The later DL102s and D4500s and G4500s have it standard. Now, if the bus is properly maintained and aligned regularly, the driver should be able to detect if the bus is pulling to the right or the left, which would prompt him to stop and check (and feel) his tires. But... If the tire was running flat (or deflated) it heats up. And it'll eventually get to a point where it ignites. And once that happens, there's virtually no way to put a tire fire out. And it burns so hot so fast that it consumes everything around it. Based on the pics after the fire was out, all six rear tires were burned off the rims. That kinda heat will easily consume a coach.
There'll be an investigation but that's what my guess is... A tire fire.
~~taking my Greyhound hat off, putting it back on the shelf~~