A sudden temperature change or uneven temperatures to the combustor's substrate can cause cracking.
One of the main reasons for this happening is refueling with wood containing some form of moisture. This can be wood that has been exposed to snow or rain, or perhaps green wood that has not been stacked and seasoned for at least one year.
The thermal shock comes when the moist smoke comes in contact with the combustor running at temperatures in excess of 1000° F (540° C).
The cell walls will develop hairline cracks and eventually pieces will start to fall off the combustor. This loss of surface area means there will be less catalytic surface space and less efficiency of the combustor.