Spotify randomly starts playing on Android 6

[Krul]

On my Nexus 5 (not 5x)  with Android 6, Spotify starts playing randomly after I finish a phone call or when I don't get answer to my call when dialing someone. It's not every time, but for the last few days  I had it 5 or 6 times

The app wasn't playing before that, it even wasn't im my list with opened apps.

Is this a bug or there is an optiona that I can't find to stop that?