I'm pretty sure it's the fuel.
The fuel in the tank was obviously heated a lot. Because the fuel tank was completely tight, the increasing pressure within it would prohibit the fuel from vaporizing, even though the fuel was heated way above it's vaporizing temperature(at atmospheric pressure). When the fuel tank gave way, the fuel instantly vaporized in the lower atmospheric pressure, mixing itself with oxygen very quickly.
It is called a BLEVE in fire-fighting circles: Boiling Liquid Expanding Vapour Explosion.
And the results can be phenomenal!