What you're seeing is an app "crashing." Apple does all it can to conceal this fact from a user but that's what it amounts to. As to the cause of the problem, it's difficult to diagnose without knowing more about the specific app you're running when it occurs and the environment at the time (i.e. other apps, state of memory, etc.) You may get some relief simply by rebooting the iPad. (Hold the power button and the screen button down at the same time until the Apple logo appears.)