I have been getting the same behavior lately. I play music for about 1-2 hours, I stop or pause the music, I go to resume playing it.. no sound, I wait approximately 2 minutes and it prompts me to sign up for Apple Music. This is very unnerving behavior, if my account is being fumbled, what's to say my credit card information isn't being fumbled or other personal information about myself.
Unrelated issue but my playlists haven't updated for nearly a week, I add music but they don't go into the playlists, I have been through hours/days of troubleshooting.
I'm afraid I really cant trust this music service, it has way to many faults, I will not be paying for it once my trial period is over. Google Play music has less features, like the ability to share playlists etc. but the fundamentals like 'playing music' and 'creating playlists' actually work, so I have decided to return to G Play Music.
Love the phones, love the phone OS - to bad the Music Service didn't pan out.