sounds like your mind was made up from the beginning to go see the gunsmith. Why'd you ask for input? Are you missing something? You pulling the trigger unlocked the action in your shotgun. The recoil of the shotgun blast opened said action, much like an inertia driven semi-auto shotgun without a recoil system to spring the bolt back into battery. If broken in enough, every pump shotgun I've owned has been able to do this. It's not as common in Mossbergs because they rarely slicken up to the degree 1300's and 870's do. Shoot and/or rack one enough and it will, all the same.
I wish I was the gunsmith you were taking the gun to, you'd be an easy hundred bucks, that's for sure.