It could have lost a tooth for one reason or another, mucked up the clearances, then as the damage from the broken pieces spread, caused the heat damage you describe. When I was doing my rebuild, an experienced tech told me to avoid made in china planetary assemblies at all cost. They are not OEM quality and have a reputation for failing. There was a video comparing OEM to the China replacement on youtube, I can not find it now. Either way not a huge difference now as even one rotation with a broken tooth probably would have required the replacement of the same parts.