I just had this issue occur this morning. There's some theory elsewhere (other GM car forums) that it has something to do with the auto-volume feature, specifically when the heater turns on very high in the vehicle (maybe that it causes the media system to think engine is going very fast and needs to compensate with the volume. This could make sense as for me it happened when it was very cold out, but there have been other days this week that have been colder where this issue was not present.
I was under the impression that the auto-volume feature was an adjustment behind the scenes, so it wouldn't manifest as if the volume were being increased manually (i.e., the volume bar displays and cannot be overridden). This makes me think that it is not auto-volume related.
It almost looks like there's some external input (e.g., a Bluetooth connected phone controlling the volume of the radio by accident?). I think it might have something to do with ice buildup or moisture.
