That looks like a classic fatigue failure since it started on an edge and is square across the face. A torsional failure would have more of a spiral shape. So you're probably correct, the shock of the reared end failure probably started a crack and then it just spread over time.

I have 5 or 6 core transmissions to pick parts from so I'm going thru them and grabbing the best parts from each. Most of them have good internals but a couple of them have toasted rear drums. Not sure why, but the rear bands are shot and the drums show signs of being overheated. So I'll probably toss those parts unless they are worth something as cores.