I bet I know what happened there, as I've had this exact same condition happen to me, with lots of shells, all because of something I was doing back in my "experimental" days of reloading.
The crater you speak of and show in your pics looks like somebody trimmed the flash holes with a flash hole trimming tool. Nothing wrong with doing that, except it really ain't gonna' help with handgun cartridges - rifle: yes, handgun: no.
Well, lemme' tell ya what those thin flash holes look like after blasting a white-hot primer through there - they look just like your pic with that man-hole sized flash hole. It just burns away what's left of that thin area of brass. This was probably due to trimming the flash holes a little too much, but regardless, trimming the holes will most certainly accelerate the condition.
Although shooting brass like this likely won't create any issues, I decided to garbage the few I had just to be safe, and chalk it all up to experience.
P.S. - Are those Winchester white-box rounds? If so, they're known to be one of the cheapest, thinest brass rounds out there for reloading, and people tend to shy away from them because of problems like premature brass splitting and the like...