I have not heard anything definite about the cause. But the new motor does come with new clamping bolts and I suspect it has something to do with how the new parts are mated and bolts torqued. I know when they first replaced my gearbox, the technician showed me some metal shavings where the parts mated which could be caused by fretting. I doubt though, that we, or even the dealerships will ever be told what the real cause was. In fact, they may still be just trying things to see what works.
In the meantime, mine is still good after the motor replacement, and hopefully it will stay that way.