this issue only occurs on low end devices with little RAM. When you leave your application to do other stuff, other applications might be forced to use the RAM needed by your game. This causes the game to be shut down, resulting in a restart when re-entering.
Unless you can lower the memory usage of your game, there is not really much that you can do, at least no with small effort. Things like saving your exact game state when the application goes into background in the database and recreating that state on resume, is clearly an overkill in my opinion, since it happens only to low end devices.