That, and the fact that it was filled with hydrogen instead of the specced helium. The Germans couldn't get helium due to trade restrictions.
Agreed. The (essentially) thermite-doped skin ended up serving as ignition source and fuel, and the hydrogen served as fuel and accelerant.
It if had just been the hydrogen, or just the outer skin burning, it would have taken *much* longer to burn. The combination of the two produced a devilish inferno.