The striker safety is disengaged, either by the safety spring or lever being out of spec, installed improperly, or just gummed up.
Then you have a tolerance stack issue between the striker extension and sear, resulting in very little contact between the two. Then you apply a little pressure or torque to the frame, which gives you another couple thousandths and voila, the striker extension slips off the sear, pistol fired.
I’ve seen striker blocks deactivated in more than one 320. The spring that is supposed to reset it back in place after firing is tiny. The lever itself is also very small.