Denon 2805 Zone 2 sound through main speakers

I just recently had the zone 2 speaker outputs connected to speakers on another floor in my house. For the first couple of days, I was able to play zone 2 with the tuner as input source and have the main amp on dbs source, for example, without any issues--in other words, I could only hear the tuner through zone 2 and could only hear the sound from the dbs through the main speakers.

Then yesterday, suddenly the sound from zone 2 started coming through the main left and right speakers as well as the zone 2 speakers. I spent quite a few hours going through the setup with the manual in hand and wasn't able to resolve the issue. I did notice that if I had both zone 2 and main input sources as tuner, that when I would power zone 2 on, the sound through the main speakers would get much louder and if I turned zone 2 off, the sound would be reduced.

I can't figure out why turning on zone 2 is now sending sound through to the main left and right channels.

Any thoughts?