This is due to the System Audio Control EasyLink feature being in use when you connect the home theater set to a HDMI-CEC compatible TV.
When the TV wakes up to a TV channel, the HTS7XX0 will wake up to the mapped source. In this case, it would be the selected Audio Input source.
If you do not wish to have this feature, you can disable it by performing the following:
- Press the HOME icon on the HTS remote control.
- Select [Settings] then press OK
- Select [EasyLink] then press OK
- Select [System Audio Control] and select [Off]