Some have mentioned tires... Could that be the case (even assuming the safety fuses had malfunctioned somehow) ? If not it is a bit of mystery...
Wheel fuse plugs are intended to protect the aircraft and ground crew after high energy braking. During that event, the primary heat source comes from the core of the wheel assembly and moves outward. If there was a fuel-fed fire on the ground, the heat and flames would come at the wheel assemblies from outside the tire moving inward - therefore the tires would heat (and burn) very likely before the fuse plugs on the wheels reached their design melt points.