A body firing problem where particles of calcium sulphate in the fired or bisque fired body slowly absorb water from the air, expand, and pop out a section of the body. These particles can be contamination or naturally occurring contaminants in the clay.
This is a common problem for people who mix their own clay bodies, using raw, unground clay materials.
As you can see from the search, this is becoming "a thing". The ash is being incorporated into both clay bodies and glazes. The ash of pets and humans. If you are a potter wondering about doing this here are a few tips. Do testing, better to use up some of the ash for that than have to throw away the ware to make! If the ash has not been ground (likely the case for pets) there will be bone fragments, these won't melt so need to be removed for glazes. For wedging into the body, testing will be needed (consider the possibility of lime-popping). Be careful to write down your procedure during testing so that production does not bring surprises. While you can add ash to commercial bottled glazes, the percentage will be low. If you make your own dipping glaze, 50% ash should be possible. Do tests without colorants to get a base glaze that is melting well and does not crawl. Add stain powders to test colors, zircon and titanium dioxide to opacify (the latter will variegate more). Color and opacifier additions can introduce crawling, test these well also. Development procedures for wood ash glazes can provide a starting point.