Mains Coming Through Mixes?

So last weekend I was recording a large band with my QU-32. I used all my monitor mixes for the band members and I listened through the headphone jack on the board. The guitarist wanted only bass, guitar, and vocals in his mix, so I set that up. He kept complaining he heard keys in the mix even though the key fader was all the way down in his mix. I wasn’t until I turned down the key on the main LR mix that the problem was solved. Similarly, when I did a play back on the recording, I pulled up the faders in the LR mix, and everyone could hear it in their headphones. What am I missing here!?

Are you recording to a DAW and have it’s output coming back to one of the boards inputs?

Yes. It’s coming back into 31/32. After I posted this, I noticed the DAW feed was coming back through the group masters. On these two mixes, the faders were up and that’s where the feed was coming from. I have not really grasped the concept/need for the groups yet. I think I have fixed my problem though!