What would cause both catalytic converters to fail on a 2005 Outback with 101,000 miles?

Just the fact that catalytic converters dont last forever. It can depend on how it's being driven too, stop and go or short trips are harder on your converters than highway miles are.

Plenty of things could have happened, or combinations. Driving duty cycle determines most of this. Short trips, numerous engine shutdowns and restarts result in many thermal cycles. When the car is cold, it uses more oil, that oil hurts catalytic converters. Then there is the rich air-fuel ratio when the engine is cold that loads the catalytic converter. These short trips, short thermal cycles just don't let the catalytic converters to get hot and stay hot to burn off and pass the loads with out accumulating on the catalytic converter. A lack of a couple good highway runs every once in a while to purge the catalytic converter can hurt. Also could have been bad fuel, contaminated fuel. It is important to make sure a car with catalytic converters does not have misfires which would overload the catalytic converter.

Check with your dealer, they have had problems. The dealer replaced mine w/o charge. The warranty should be 10-15 years on those.

I think the federal required minimum warranty is specified in years, not sure how many. Check the owner's manual, the info is in there.