Not quite sure if this a problem with Shuttle or with Samsung's strict RAM management (Galaxy S5, original unbranded Switzerland ROM).
When having Shuttle open in the background (music is playing) and opening another app, playback will stop and Shuttle will close. The icon in the notification bar reappears after some seconds, but playback is paused at the same position as when I opened the app the last time before the crash.
Sometimes Shuttle even stops when I just return to my home screen (using the home button). It then reappears within the notification bar as well.
All this is happening at any time I use Shuttle, but not always instantly when opening another app.
For that reason I switched to Poweramp for the last week and there was no such strange behavior at all.
What I still have to test is if this is happening with the Shuttle stable release as well. I'll report back.