Muskeg13,
Hopefully someday the cause of this incident will come to light. It will make me more conscious about the way I load the ammo for my lever guns. That's for certain.
Take your Lee Auto Prime tool apart and inspect the bearing surfaces. If they are worn replace them. Then keep all bearing surfaces lubed. Also I seat my primers twice. Seat them once, rotate the case 180º and seat it again. That way I make certain it's below flush all the way around.
When inspecting seated primers I use the tips of my fingers. Been doing it so long they are educated to what a below flush primer feels like. But as I've gotten older I've found my fingers seem to loose sensitivity after a while so I have to prime smaller batches.
Joe