He thought maybe he was inadvertently pulling the slide open because he wasn't expecting that much recoil, so I tried it. I rested the gun over the "V" of my pointer finger and thumb, not holding the forestock at all. The action came all the way open and ejected the spent casing for me as well.
Now we aren't sure if the barrel is keeping the bolt from fully locking properly, or if this is a side effect of the Winchester "Speed Pump" feature (I never really knew what that feature was supposed to do anyways), or if it is a matter of too much shell for the gun to handle?
Each shot was dead accurate at about 70 yards, and it didn't seem to be an "unsafe" condition as it was firing accurately and ejecting the spent casing, but it could definitely catch you off guard in a hunting situation if you are used to doing all the work of pumping a new round in.
I've fired everything from light target loads to heavy 3" magnum turkey and goose loads to rifled slugs through my Mossberg 500 and never had this happen, so I can't image a 2-3/4 slug being too much for a 1300 to handle.
My initial research on the web found another guy in a forum that this happened to, but haven't been able to find too much more info on it. I'll have to keep looking, and in the meantime he was going to have a gunsmith look at it to see if there is an issue with the bolt locking mechanism.