parking brake?
I assume the vehicles have parking brakes? Transmissions that can be parked/set into low gear? I wouldn't simply assume that cargo shift is the cause. A pallet on coasters, would be thinkable. But that's not the case here.
Furthermore - once the plane was in a 90 bank, nothing would have prevented rear loading from rotating the empennage rapidly downward. That didn't happen.
Third - probably this problem was present at rotation since the wheels were down the whole time. Vehicles crashing rearward at or before rotation would have immediately told the crew to abandon any hope of a successful departure.
I'm thinking that this points to a problem with the airplane itself.