Have you ever tried to replicate the situation by doing a “virtual soundcheck” using the Qdrive ? That would allow you to monitor what’s happening without the live-stress.
Are you using a lot of compression on the mains ? If the treshold is around the 10 mark you can run into what you’re desrcibing.
Do you have lot of spill from the wedges into FOH ? A musician deciding to back of + you pulling him/her back a little goes double that way.
Giga