Why the fire bottles failed to confine the fire to the nacelle is a big question -
but we don't even know if it was a nacelle fire.
Quite possibly the fuel simply leaked while in a slipstream environment and wetted a lot of surfaces behind the nacelle in flight before dripping down and wetting the nacelle and the ground after it stopped and only then the ignition.
It's gonna be an interesting investigation to determine the links in the chain