Same issue. I'm also on Nexus 5x but the same thing occurs on my LG G2. I tried all the fixes in this thread but nothing helps. Several times a day Spotify will pause for no apparent reason, but only if I'm listening online (offline works fine).
I've had this trouble for the past month suddenly. Notifications come through and I have to go on the app for it to play again and most of the time I have to take it out my pocket and unlock my phone. I've found that clearing and uninstalling doesn't work. The best way if your using it on wifi is turn power saving off and if it still happens then I'm sorry but spotify need to **bleep**ing sort it out lol.
oldtrees tip also works where the same Spotify user was logged in under two different profiles on the same Android tablet. Music playback kept pausing until we logged out of Spotify in one of the tablet user accounts
I've checked my account elsewhere and it isn't being used at work. I've tried logging out everywhere via my account online and that hasn't worked. I've even changed my password and it's still keeps pausing. Definitely no one else is using my account. It's really very annoying