I havent found a way to fix the same issue im having without installing a 3rd party app or disabling the apps that are starting.
I did fix mine though. I only use pandora and hardly ever use google play music app. So I went into setting, apps, selected the two music apps and the sound recorder that was starting when I connect to my truck radio via bluetooth, and disabled them. It does not uninstall but, it does delete you app setting and saves, not your music or app. Now when I connect it does nothing unless I start the app.
What is really causing this is, when you connect to a bluetooth media device, ie. Car stereo, headphones, speaker, home theator, that device has a setting programmed to automatically send the play command to your phone. Some devices you can disable it, some you cannot. Basically when you connect the audio device it is automatically pressing the play button for you. If you already have a music app open then it is suppose to just resume or play that app. Sometimes it glitches and does it for others even if its already open.
The issue is when you have no music apps open, you connect to your bluetooth device, it sends the play command, so your phone is opening any, all, or one music app that accepts the play command.
It sucks but maybe this will help someone else figure out a workaround with disabling or installing apps.