three possibilities on the firing pin strike. I've seen overpressure totally flatten out a primer strike during a kaboom.

second possibility is something other than the primer set it off, either an ejector, extractor or seated mag

third possibility is the chamber was hot enough to cook off the un chambered round.

One thing is definate, that round was NOT fully chambered when it went off. Based on the odd rifling marks, I would suspect the nose of the round was in the chamber and the rest of the bullet was not seated. Inserting a new mag and cycling the action probably caused something to strike the primer area enough to cause the OOB discharge.