What would cause a catalytic converter to fail?

Your catalytic converter is rarely if not never naturally damaged. Reports of failing catalytic converters due to bad quality manufacture are almost never accounted for.

The failure of your catalytic converter is most probably a failure of another part of your engine which will consequently not transfer to the converter the amount or the type of substances it is supposed to process. If your engine is out of tune for example it will not handle the correct mix of fuel and air and that can result in damaging your converter. If the fuel is pouring in the converter it might create an overheating situation that will melt the metals in the exhaust system. There has also been reports of oil or antifreeze liquids entering the exhaust system, which will clog the converter and reduce the power of the engine as well as generate an unusual pressure in the system that can damage other parts of your car. You also need to monitor the state of your O2 sensor to make sure the mix entering the catalytic converter is not incorrect - that would have the consequences stated before.

Last but not least, the catalytic converter contains a ceramic that is rather fragile. If something damages the underneath of your car (accident, rocks, obstacle) it might result into a loose catalytic converter that might break, the pieces of the ceramic will provoke a complete dysfunction of the filtering capacities of it.