[yarddog]

I once had one of the little Beretta Minx .22 short pistols go full auto and it enptied the mag so fast it was like one short roar.

Interestingly, I never did figure out how and it never did it again.

[B4Ranch]

I just disassemble it and using a fine stone, put the sharp edge back on the sear again.

In your case, I am leaning towards a piece of sand or powder got hung up on the sear.