The force of the recoil is ejecting the shell because you aren't holding the action closed when shooting from a rest..simply Pulling the trigger and firing is disengaging the bolt lock. This is because the gun is well broken in. This is normal. If he was chambering a round, not firing, and pulling the forend back and it opened, this would not be normal.
Wow @ this thread
This happens on Winchester 1300s, 870s AND Mossbergs...I've owned at least 3 or 4 of all makes. If the trigger being pulled didn't disengage the bolt lock, then what does???? What other manual operation is involved other than firing and racking the slide? Why does holding the slide release down and racking the gun allow the bolt to unlock and slam rearward ejecting a shell, spent or otherwise? Think a little bit here, please. LOL
This very occurance is actually a measuring stick for me for a well broken in pump gun.