Memory management on Android is awful. My music player routinely gets killed; I've tried all options available to me to prevent this.. It's just too difficult for Android to work out "you're listening to music, even though there's no visible ui, so I won't kill it". I understand that there is a whitelist of music apps which don't get killed - so intractable is this problem - but the one I use (Rocket Player) isn't on it.