There are a few occasions when this might happen. Android can decide to shutdown the audio service at any time it needs resources and it'll try to restart it again. This is rare, but can happen if you have a lot running on your device.
Alternatively, the service can shut down if it encounters an error and can't continue playing. Android tries its best to detect this and will try to restart playback within a few seconds.
As always, we're continually improving the application as a whole and will do what we can to improve and help prevent situations when either scenario can occur.