OMNI Software Mixer Problem

I was mixing this morning and realized I was getting distortion on the left channel. Did cable swaps and the problem followed over to the right side. Then sent material to discrete Left and Right, and the problem only existed in the Left output. On a hunch, I disabled the Optical input in the software mixer by deactivating the MON buttons, and boom, the problem disappeared. Clicked it back on, and the problem came back.



I swapped optical adat cables, and the problem seems rectified. BUT...the troubling question is, why was the audio coming directly out of my ProTools session affected by the software mixer?



Here's my rig: Mac Pro Mid 2012 40GB RAM, HDX2, OMNI, Pro Tools HD 11.1.3, OS X 10.8.4 (with a co-install of PT HD 10.3.8 that I rarely use).