Never seen that! I'd speculate that a void was present in the swaging stock, and ended up pretty much centered in the finished bullet. So the "bubble" actually shot out in two or more pieces. But I'm only guessing.

I'm not sure how they make the stock that gets swaged. If they roll it like taffy (using metal plates, not hands!) I could imagine a circumferential unbonded layer. But again, just a guess.