Maybe the abrupt arrival caused the main transmission to be dislodged and somehow ruptured a fuel line. Even so, if faced with a lack of drive you'd think they would have rolled off the throttles and turned off the fuel boost pumps before landing. So there would be very little fuel to burn upstream of the fuel tank. But there would still need to be an ignition source. It's pretty difficult to set fire to kerosene even if you put a match to it. Another observation, the fire burned all the way through the cabin floor and fair chance compromised the fuel cells. In which case I'm very surprised the whole thing wasn't consumed by fire. Couldn't have been very much fuel in those tanks.