Old thread, I thought I'd chime in here again. This problem was fixed with a fairly recent update several months back to the Google Play application. Unfortunately, in the most recent update which was just within the past week or so the problem has returned.
This is an issue not just with Bluetooth headsets, but also with cars and apparently as the OP initially indicated can also affect Android Wear watches. For me, the problem surfaced when starting my car and all of a sudden Bluetooth sent a signal to start Google Play Music. Obviously I would have to pause play or end the Google Play music application to stop the audio. Then I discovered the same thing would happen after I ended a call if on a Bluetooth device whether it be a headset or the car.
The big issue for me is that sometimes I need to turn the volume of the phone call up louder to hear whoever happens to be on the other end, and when the call ends I'm suddenly blasted out of my car seat by Google Play music. It has also nearly caused my eardrum to be punctured when wearing a Bluetooth headset and suddenly getting ear deafening audio shoved down my ear canal.
I've been on with tier 2 tech support at Verizon and they are aware of the problem and say this is definitely a Google Play problem, not specific to any one phone model. You may or may not experience this, but if you do the solution is just as mentioned above, to uninstall the update for Google Play music. You should also then turn off auto updates for that app within the Play Store.
I just uninstalled the app's update, tested and found that the program was no longer auto starting. Then I installed the update and received a phone call, and after the call ended sure enough, Google Play Music Started and played a song I've never even played before. You would think that autoplay would be a user-selectable option within the settings menu on Google Play Music but unfortunately there is no such switch.
This needs to be fixed since it would be the kind of thing that could potentially cause harm if someone is scared out of their wits by their car audio system while driving at highway speeds and as a result of the distraction while frantically fumbling for the volume control they were to lose control of their vehicle. I sure hope Google fixes this quickly.
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