You got me curious, so I loaded up a few more CCI 400s and 40 gr Vmax. Nothing like I got the first time. Although there was a small rim around the firing pin dimple, it sure wasn't what I'd call an over-pressure sign. That first series of rounds must have had particularly soft primer cups. It happens.
It even happens to the commercial mfgrs. Had some Hornady rounds punch through. They sent me a new box.