Jim's assessment is pretty much spot on. I've seen it happen with GI ball and equivalent handloads, and the common thread almost always includes a weak or short firing pin spring.
I've staked the edges of firing pin stops as an expedient measure, but prefer a fitted stop. No need to make it so tight that it requires a punch and hammer to move it, though. Just enough to keep it from falling from gravity is all that's needed, and using a punch or the tip of the hammer strut to pull it down is acceptable.
The side benefit of using a fitted stop is that it makes for more consistent ejection by keeping the extractor aligned.
If your front porch collapses and kills more than three dogs...You just might be a redneck