Surround channels still active when on 2 channel source?

I was putzing around last night and I left my DVD player playing and switched over to my CD player. The DVD was using the 7.1 analog inputs but switching to digital input made no difference. I switched to the CD input with nothing playing and I could still hear the surround content of the DVD playing. What the heck? Shouldn't unused channels be muted? Why is my CD (set for straight 2 channel analog) allowing this to come through.
In practice of course you would never have this situation but I can't help but believe something is wrong. Turning the 1070 on and off didn't help and changing inputs didn't help.