Most often, the problem of glaze crawling is due to poor or a compromised adhesion of the glaze to the body. Because commercial prepared glazes contain significant gum or binder they can be layered and yet still stick and dry well. Usually. But this time the body is a fritted porcelain, its surface is smooth and dense because it was bisque fired too high (1850F instead of 1550). While the dried glaze surface was free of cracks and appeared stuck on well, its adhesion with the body was obviously not good enough to withstand the rigors of being firing on this smooth and vitreous surface.
A ceramic glaze fault that occurs during firing of the ware, islands of glaze form as it crawls, leaving bare patches of body.