What would make a catalytic converter go bad.

Catalytic convertors can need replacement for a couple of reasons. They can loose the capacity to store oxygen which is its main function, it can plug up, or it can crack the inside and rattle. The main reasons for failure are coolant or oil being burned by the engine. Engine running rich on fuel due to bad O2 sensors or fuel pressure regulator among other reasons. Most of the time the crack inside by getting good and hot while driving and then driving thru a puddle and the thermal shock may crack the catalysts. When replacing convertors alway try to determine if something caused it.