A bit more fact-free speculation.
As the problems appear to have occurred at about 5000' AGL if it were brought down by ground fire it would need to be a weapon effective at that distance from where it was fired, that rules out rifle fire, don't know the effective range of the Stinger and such like.
For that amount of disintegration before impact, if it was an on board bomb, I think it would have to have been a big one.