What are the most likely causes of catalytic converter failure?

Thanks ziptie. I'm not sure how it failed but the service engine now light came on and the I believe the shop diagnosed it using the computer interface but I'm not absolutely sure of that. And it was only a code P0420. Can you please clarify what this code is since it is not from the diagnosis?

P0420 is simply a code that is set when certain conditions are seen by the PCM indicating that the converter is below its intended efficiency level.

Thanks again ziptie for the clarification. Sounds like I should not only find out if they diagnosed the cause but also get a second opinion.