I’m leaning towards a loose neck and bullet setback creating over pressure. Far more damage done to the base than what I can see as just case separation.
But who knows.
Did you miss the part where he bent the charging handle "mortaring" it out? A lot of that damage was post-mortem.
Have you ever seen one mortared that pulled off more than the rim? I haven’t. What took out the pie slice shaped divot was not the extractor. In my mind it took the easiest route and gas escaped through the portion of the rim least supported, the extractor cut.
