what causes the catalytic converter to go bad?

That would be very fast for the catalytic converter to fail. Usually a converter would need to be replaced due to wear. What is the fault code from the check engine light?

It's still under warranty, what does dealer say??

Did the dealer tell you you had a bad catalytic converter? Converters are covered 8 years our 80000 miles. They usually fail because the get to hot from engine misfires.