The explosion could've been caused by a number of things, catastrophic blade failure, the sound of the engine breaking loose from its mount, fuel line rupture causing the fuel in the wing to ignite when it came in contact with the combustion section of the engine, etc. etc.
The fires aren't even out yet and we have the amature "Accident Investigation Board" here on this site digging bunkers in their backyards, sand bagging their porches, and breaking out their tin-foil hats. Before you folks start rumor mongering, why don't you let the professionals do their jobs and when they come out with their findings, then you can talk about that! GEEZ! Gimme a break!