I noticed a similar issue the other day, but with voice navigation. I usually set the nav voice volume (via the touchscreen) at 2 or 3. I could not for the life of me figure out why that volume kept changing. I realized that occasionally I would try to raise or lower the radio volume with the scroll wheel when she (nav) was talking, and if that's the case then it changes the nav voice volume instead of the radio volume.
Strange behavior for sure. I can see how someone might want that momentary control of the nav voice volume, but generally it seems odd to have volume control switch around to whatever is active at the moment. It would make sense to me then, that when you are trying to reverse and sound/volume priority is given to the parking sensors, that you can't mute the radio because the car thinks you are trying to mute the parking sensor sounds. This is pure speculation but would make sense to me based on the nav volume behavior.
Simple solution would be to have scroll volume 100% dedicated to radio (or phone call, if on a call). All other volumes should only be controlled via the touch screen since presumably you don't need to fiddle with those other features that often or ever.