Cause: This behavior is platform specific as Apple does not allow all apps to exit completely. Apple designed multitasking in iOS to manage itself. What that means is, they do not want users to manually exit an app from the app level because they want all apps running in the background to allow quick access to them when needed.
There is an informative discussion on this topic in the Apple Support iPad forum:
Resolution: To close the application, use the iOS device's Exit button on the upper left corner of the screen.