I had it happen once a few years ago. I should have noticed it over the summer when I was doing some load work and I had a couple that needed a second strike. I figured I somehow screwed up and didn't seat the primers fully.
When the weather got cold none of them went off. I replaced the striker spring and end of problem. FWIW, the old spring was clean, so over-lubing was not the cause. The rifle was over 30 years old and stored cocked. I now take the tension off the firing pin before storage.
