If you set up your Comcast remote to control On/Off for television and the Sound-bar using the correct control codes for each, it is possible to have your problem.
You need to aim the Comcast remote at each component: Cable-box, tv, and sound-bar. If you missed the sound-bar it would still be "Off".
Hitting the All On button on the Comcast remote again will now turn "off" cable-box and television but turn "on" the soundbar. Correct the problem by turning "off" the soundbar with it's remote or on/off button.
If the soundbar was directly connected to the television instead of the cable-box, it would most likely turn on with the tv when cable remote turned on cable-box and tv.