Online games are extremely sensitive to the quality and speed of internet connection. Moreover, they require verious factors to work smoothly altogether.

Any sort of disconnection (losing a stable connection, switching to other apps, checking emails/notifications, and even going to the device's menu) causes the app to consider player as inactive and to abort a started game. 

Thus, to avoid the issue, please, ensure the internet connection is stable and fast, and do not collapse/close the app during the active game.