This can be due to a number of factors, so first a bit of analysis:
Was it during the bisque firing or the glaze firing? If in the glaze firing, if it occurred during heating the glaze at the edges of the crack will be rounded, or may have run into the crack. If on cooling, when the glaze has already set, the glaze at the edges of the crack will be sharp edged.
- It was already there! Perhaps it occurred in making the piece and you didn't see it till bisque firing, or in the bisque firing and you didn't see it till the glaze firing
- Poor joins. If joins are not done well then they are a source of weakness, and may be the first thing to go during the stresses of a firing
- Thermal stresses. As the pot heats up and cools down, different parts will expand or contract by different amounts, causing stresses which may cause cracks if large enough. This may be due to the design of the piece, or temperature differences during the firing which may be due to the way the kiln is loaded or the firing program
- Quartz and other inversions. Quartz and some other materials change their structure at given temperatures, causing stress on the pot as it tries to accommodate this. This is particularly a problem if the change in volume is large, or if it happens quickly.
- Uneven glazes. As the piece cools down, the glazes will first set hard, and then contract. If they contract by significantly different amounts in relation to each other, they will put stresses onto the pot, which can result in cracks or breakage. A good example of this is if the piece is just glazed on one side.