How to keep audio players from starting automatically upon reconnecting to a bluetooth device?
Due to poor development with some phones, most music apps will still be running in a background process even after you close it. 

My advice is to use an app called "Purify". Unlike most task managers, this app keeps app processes closed after you close the app.

But be aware that Purify can temper with many of your social media apps and you may not be able to receive notifications from Facebook or Instagram after you close the apps.

Purify SHOULD be able to prevent your audio player from automatically opening when connecting bluetooth.

Also, it works BEST on a ROOTED device.

If it doesn't work, let me know!