The choke section should never act as a restriction to properly functioning shot pellets. Rather, it is a constriction which the shot will flow through with decreasing pressure and increasing velocity. I think the disproportinate number of fowlers, as compared to gameguns, with bulges immediately behind the choke section is evidence of the larger shot usually used for fowling being more succeptible to bridging. The even brief existance of a bridge could slow the shot/wad sufficiently to create a gas hammer behind the wad. Sounds like the same thing could be happening here.

Is there an inspection that would catch a fragment, say base wad, from the first firing? Are the hulls tracable to the gun fired in?

Dig, were both barrels bulged?