catalytic converter plugged on 01 3.8L

[hawkeye1956]

At 63000 miles, the catalytic converter plugged on my 01 Impala. The only symptom was that it was low on power which also caused the transmission shift points to be delayed. Fortunately, this is covered by emission control system warranties that all manufacturers are required to honor up to 80000 or 8 years. In my case, an independent garage did the work as I didnt know I had a bad cat when I took it in but GM is going to reimburse for the full amount. Now that it is replaced and the car is running great again, my concern is that it will happen again as no other problems were diagnosed. The car gets great gas mileage (30+ on hiway) which would seem to eliminate fuel problems. There were no service engine lights and my mechanic assured me that they checked everything out and no other problems are present.
If there is no other problem, why would the cat be plugged so soon? Should I assume that there is still an underlying root cause that will most likely cause this to happen again? Thanks for all replies.