Over the last couple of weeks, this has happened three times on my Android device and had never happened before that (using Android devices with Spotify for years prior).
It has happened both during a phone call and a Skype call that music has started playing through my device. Fortunately the volume level has been ambient enough that it's just made for interesting background music, however, when I paused and closed the application during the Skype call, it killed the audio on the device and meant that we had to restart the Skype call in order to hear each other again. This was a work related Skype call, was very embarrassing, and if it happens again, I'm reluctantly going to have to uninstall the application from my phone and downgrade my account.
I suspect it may be related to this issue where Spotify is changing from device to device, which also started happening for me around a similar time. In both instances, Spotify was open on a Mac within the same wifi connection but also in both cases was in a paused state while the phone conversation was happening.
I should also note that I have a large number of playlists containing thousands of songs that I've built over the years, and whenever I open Spotify, I have to leave it for approx 2-5 minutes before it's usable. This is despite the processing power of my device and the relative speed and responsiveness of other applications on it.
Spotify Android Version: 184.108.40.2069 armV7
Android Version: 7.0 NRD90M.G935FXXS1DQCE
Android Device: Samsung Galaxy Edge S7
Mac OSX Version: 10.12.4 (16E195)