Spotify constantly pauses when listening through headphones on iPhone

[MLW2]

I use Pioneer headphones with my iPhone 6 and when listening to music from the Spotify app, it will constantly pause and then start playing again - mostly this happens when I put it in my pocket / face the phone down in my hand. Sometimes the track will pause and then won't start playing again until I manually press 'play' again. I have uninstalled and reinstalled the app and am still having this problem. I have found that this only happens when I am walking somewhere as well, if I am using Spotify when I'm idle it is fine. Is this a problem with Spotify or the iPhone itself?