you don't necessarily need to strike a primer to get it to go off. if it was pressed hard enough I believe that it could go off. I say this because when I first started reloading, I had a primer that go fed incorrectly. as I tried to pop it out with a pick, the pressing on it caused it to go off. No biggie, no powder in the shell, but it went off. Might be what happened in a round about way. Could have been pressed up against something. Just a thought. Maybe not though.