Oil Drain bolt loosened on its own?

[xdarkxfirex]

I always do my own oil changes. This time when I was about the change my oil, I found my drain bolt came off with no force. I used a larger 14mm gasket last time since it was all I had and it sealed, and I used to proper one this time. Luckily it didnt leak at all.

I never had one of these bolts loosen on there own. I tighten them pretty tight with a 3/8" ratchet. I think the bolt was a little deformed with taking it out in a vice last time. What could be the cause?