Nobody else has replied, so I must step up to the podium. Water got under your shell's Bormann fuze, and froze, probably many times, which pushes the fuze further and further up above the fuzehole with each freeze. Internal gas pressure doesn't have enough force to do that. As most of us already know, freezing water in what begins as a very tiny crack can eventually split granite. That's also what caused the greatly-expanded lead sabots on some Gettysburg Monument Hotchkiss shells, and the mysterious "60-caliber" Gardner minies. Jim and Dean Thomas actually conducted a refrigerator freezer test of my freezing-water-in-a-crevice theory of how some unfired Gardner minie bases got expanded to that oversized diameter, and confirmed the theory.
Regards,
Pete