The best guess on how it happened is that the wad from the previous round got stuck in the muzzle area of the top barrel, and was never noticed by anyone. Neither the shooter, trapper or squad-mates. It was the last shot of the station. When the shooter shot the first pair at the next station, at the second shot, the choke tube of the top barrel flew off to the right and the resulting muzzle damage in the picture was observed. Shooter was using factory Federal Top Gun loads.
Shotgun Report® 
No expert here, but I agree with D. Goddard above. I would put a sizable wager on the ballistics investigation finding that the screw choke was loose. Either that or the choke tube had gotten damaged and the skirt was sticking up enough to cause the shot and gases to get behind the choke tube. Those appear to be Trulock choke tubes which are quality product. I believe what is most telling is that no one heard any off-sounding shots just prior to this happening. In my experience, I have NEVER had a wad stick near or at the choke. Always near the chamber forcing cone. I have also never experienced a wad sticking in a barrel due to a factory shell not performing properly. Not saying it is impossible, but certainly very unlikely. Factory ammo is so darn reliable nowadays that it is hard to imagine this being the ammo’s fault. Fortunately no one was hurt!
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Looks more to me like the choke tube wasn’t screwed all the way in (or worked its way back out) and the shot caught it.
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Pressure is pretty low at this point on the barrel .Hard to believe a plastic wad stuck at the end of the barrel would not be blown out by the ejecta. A blockage for sure but it must have been pretty tight or heavy and stuck. Ballistic experts correct me if I am wrong. If everyone is safe, not all is lost. Time for a new gun.
Dk
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