There has been a lot of chat about how this could have happened, both an unlocked reverser or asymmetric thrust could do it ( although fadec would probably limit thrust with a deployed reverser , been a while so not sure on that one ).
Most accurate posts have suggested VMCG and governor failure. A combination of these is almost impossible to control. Usually a governor failure involves an increase in thrust as the engine "runs away" before the fadec shuts it down. As the thrust runs away the pilot instinctively corrects the yaw with rudder. So now we have, let's say right right rudder in, as the right engine runs away. Then fadec shuts it down. Now you have right rudder in with a right engine failure, end result below VMCG is exit stage right.
Not saying this happened here, but any asymmetry below VMCG would be similar.