At low temperatures glazes are not stuck on nearly as well as for stoneware and porcelain. One of the ways to compromise the bond is to put a layer of refractory material, for example certain underglazes, between the body and glaze. It is kind of like painting something that is dirty, the paint won’t stick. Certain ceramic colours are more refractory, that is, they melt at much higher temperatures. Because no melting is occurring they adhere poorly. Some underglaze manufacturers compensate by incorporating Frit in and underglaze composition, they treat each colour as an individual and adjust accordingly. Other manufacturers do not do this, they simply have one base medium recipe and add all the colours to that. In this case the bond between the Glaze and underglaze is better than between the underglaze and the body so that is where is flaking off.
A ceramic compound meant to be applied to green or bisque ware and covered using a transparent overglaze. There are good reasons to make your own underglazes if you are in production.