I have no clue what might have done it.

I have seen such things happen, but there was always an outside source. My boss at the old gun shop had a few shells get wet during a duck hunt and got some rust on the brass.... took one back to the gunsmith shop and put it on the wire bristle brush and somehow managed to cook it off in his hands. Didn't even burn him... I guess the powder was wet or something.... pretty lucky in my eyes.

I'd suggest keeping your ammo in a better place. Luck is often fleeting.