I don't see any technical connection between switching on that mixing desk being connected to one of the Main Outs and the signal at the Monitor Out. So, I would not draw that conclusion without further verifications. Unfortunately, your "trouble shooting process" didn't include a lot of trouble shooting:-)
Have you verified what happens, if you disconnect the XLR cable from the Main Output or switch off the mixing desk?
Was perhaps phantom power switched on at the mixing desk?
Have you checked the cable between Monitor Out and QSC K12? The Monitor Output isn't balanced, so a TS cable would be sufficient.
Have you checked the gain settings at the QSC?
Have you checked grounding? Which ground lifts were activated at that time?