DIAGNOSIS The soundbar's 'Auto Power On' or 'Auto Device Detection' feature is misinterpreting another active signal.
FIX Disable the auto-switching function. On the soundbar remote, press and hold the 'F' (Function) button for 5 seconds. This toggles 'Auto Device Detection' off. The front display will show 'A.D.D. OFF'.
IF THAT DIDN'T WORK The issue may be originating from the TV. On your TV, disable any 'Auto Power On' or 'External Device Wake' features related to HDMI-CEC/ARC.
HEADS UP With 'Auto Device Detection' off, you will need to manually select 'HDMI' as the input using the remote after turning on the system.