I think Daryl has it right here. As V. M. Starr wrote, in The Muzzle Loading Shotgun: Its Care and Use, "It is also good practice if it so happens that you fire one barrel several times without firing the other, to check with your ram rod and see if the shot wad is still tight in the unfired barrel, a half dozen shots or so will sometimes loosen the wad and let the shot roll out and spoil your shot right when you need it most." It will also, occasionally, blow out the barrel as shown in the picture.

As a kid, I visited Starr several times. He had a number of barrels that had been blown out in this fashion, as I recall each had the left barrel blown. He gave me one such gun to use to practice cutting a jug choke; the barrel was blown out about the same, and the hammer nose split a little worse. (The load in that barrel had been a ball seated between two wads, on top of about 5 drams of Fg. The ball had apparently moved forward, and blew the barrel when fired. Starr said this showed why you should be careful about to whom you loan your guns.)

In my experience, a couple of other suggestions seem unlikely. When a twist-steel barrel such as this fails due to flaws, it usually "unwinds" where it was wrapped around the mandrel in forging. Smokeless loads tend to shatter the barrel all around the circumference (except sometimes where the barrel is soldered or brazed to the adjacent barrel).