HP rear? It looks like the gears spread some and walked the pinion high on the ring (lots of backlash direction) and when they were out to edge of tooth, it couldn't take it and let go.
Some guys will tweak setup for HP rears to run bare-minimum backlash and heavy carrier bearing preload to try and deal-with/prevent that.
The oddity to me is that the pinion breakage looks like it was spinning rear-axle direction when it broke, but a couple of the ring teeth look like it was spinning front-axle direction. Total Newb may have called it there, knocked off a chunk and proceeded to digest that chunk through the rest.