A hard case!

Case hardening is caused by too rapid increase in cooking temperature, particularly early on in the cooking process; it is better to increase the temperature of the curds/whey mixture slowly soon after cutting. If the initial rate of increase in temperature is excessive, it can lead to increased syneresis at the outside of the curd piece which causes a dehydrated protein layer inhibiting further movement of moisture out of the curd piece and thus acts to reduce whey loss. Thus (and paradoxically), excessive cooking can lead to poor overall syneresis and high moisture in the cheese! Case hardening can be avoided by reducing the rate of increase in temperature during the early stages of cooking and/or by introducing (or extending) a healing time between cutting and the start of cooking.
Further reading:

McSweeney, P.L.H. (ed.) (2007). Cheese Problems Solved. Woodhead Publ., Cambridge.