From Xojo Documentation
The app is being closed by the OS.
The OS will close an app if the device is running low on memory. If can also occur if the user force quits the application.
The idea that an app quits on a mobile device is not one the user should be aware of. This means that in the Closing event, your app should store in a file any information about its current state so that when the app is relaunched, in the Opening event, you can restore the app to its previous state. Keep in mind that the amount of time the OS will give you to store your current state in the Closing event will be very limited.