Maybe your muffler is creating enough back pressure you rear o2 sensor is now detecting a change in exhaust gases. Rear sensor is there to monitor the system, compares readings with the front o2 sensor and trips a code if they are getting the same readings. The idea is the rear sensor will help identify if the cat is not doing what it is supposed to do. I gutted my cat an no code was triggered so I guess I was lucky.
Just a stab in the dark...
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