The case is sealed, using a fairly rigid material.
If it cracks, environmental contamination can be exposed to the die, but more importantly, a bondwire (trapped in the encapsulant) has probably been broken in the process, and now the part is useless.
This is unusual to happen, even with soldering heat; but some conditions can cause problems. For example, if the material tends to absorb moisture, which is then released rapidly at high temperature, it can crack apart. Slow heating, and preferably a bake-out cycle (modest heating for a longer time, in a dry atmosphere), avoids this.
Tim