This may happen if the firing temperature is high enough for the clay to be softened, and so it can deform rather than crack. As such, it is most likely to happen during the glaze firing
Possible causes are:
- Thermal stresses from the design or the firing, including stresses from shrinkage of the clay
- Physical stresses. For example, if firing a piece on stilts to too high a temperature it will deform around the stilt, which will be impressed into the clay.