There was an accident to a Skyvan in Oman doing something similar to this.
They were either planning a low level extraction or, given the load of fuel drums, a touchdown and rapid offload.
Anyway the Loadmaster was ahead of the game and had the cargo unfastened and ready to go when the aircraft came under fire on the approach. The pilot initiated a steep climb out but the carge shifted giving an uncontrolable rearward shift of the CofG with predictable and tragic consequences.