HDMI CEC control is stupid. It's on almost every single device from most manufacturers and is typically ON out of the box. Oppo Blu-ray players, offer it, but when you buy them they are OFF by default! Very nice.
Denon receivers, and many other devices support CEC and have this capability turned on. So, you want to be really sure to turn this feature off if this is not something you want to utilize. IMO, it tends to really suck, so find out where it is on your product and disable it.
HDMI Control: Off/Disable
HDMI CEC: Off/Disable
Suprised you've lived with it for five seconds without asking here...
If you did ask here, then I would have been able to tell you what was up and how to fix it had I seen it.