If you have a YouTube video playing in the foreground and attempt to play a song through Spotify (for example via the mobile app), it will cause the YouTube video to pause itself. This is the expected behavior. 

However, if Spotify is NOT playing any music in the background, YouTube will still pause the video for seemingly no reason. Then, if you run logcat via adb, you will see that (despite not playing anything in Spotify) Spotify sent a notification to the system:

D/BuellerDeviceService( 1100): isFireTvNotification(): recommendation, id=847607495, title='Spotify', passing on...

This should no be happening. If I'm not playing any music in Spotify, then there is no reason for it to be sending notifications. In the meantime, I have to manually kill the Spotify daemon to avoid the random notifications that pause apps such as YouTube.