The issue is iPhone 6+,7 across different apple iOS versions all of sudden the music starts playing in the iPhone. It is just annoying at this seems to happen at any place may be while in meeting or driving a car. If you are too facing the problem you may wish to try below steps.
Fix 1: Defaulty Headphones
If so the remote in the headphones is acting up. Is the problem occurring only when the Headphones are plugged in, If so the remote in the headphones is doing the action. If the controller of your phone is not in contact with anything and still you are seeing the issue it could be an internal fault in headphones? Suggest you to simply buying a new pair.
Fix2: Change App setting
It’s a Possible solution: Suggested App may result in opening the music app so better you try to Turn off "Suggested Apps". To turn off app suggestions, you can do this by going to Settings -> General -> Handoff -> turn off switch labelled "handoff" or 'Installed Apps'.
Fix 3: Delete Pandora App
If you are using Pandora app than this could be of very common reason for auto play of music in iphone. Try deleting the app if possible do it when automatic music is playing in your iphone 6. If the music stops it means it was Pandora app which was the cause.
Fix 4: Music library
Open the music app on your iphone. Open library in that at bottom left. On the top right of screen tap edit button, uncheck everything. Check if auto play music stopped.
If you do not wish to delete Pandora than you may try to open Pandora, whichever music app is used for music , using that play a song, then stop, and that should solve problem.