Most likely it was a poorly or incorrectly prepped vehicle or item of ground equipment pouring excess fuel into the cargo compartment.
Happened to me many years ago on a C130 resulting in having to do an overweight landing back at Richmond. Prep instructions said to drain the fuel tank to less than 1/4 full, not 1/4 less than full! End result: Fuel all through the cargo compartment. I was somewhat annoyed about it when I got on the ground, if I recall.
Inexperience and carelessness unfortunately (and this type of incident in particular) rear their head a little too frequently in the military IMHO.