This often happens when apps run out of RAM or crash for other reasons. To a great extent it depends on the app, but it can also be a sign that the iPad's RAM is running low, or the system is struggling. If it is happening a lot you should try restarting or reseting the iPad and see if that helps. If it is app specific (that app keeps doing it) then you should try clearing that app from RAM.
In case you don't know, here is how to do all those. None of them will damage or remove content from the iPad.
Close/Clear the app: With the problem app closed, double tap on the Home button to get the multitask bar. Tap and hold on any icon in the bar until they jiggle, then tap the minus sign on the problem app. This will remove it from RAM and insure a clean start the next time you launch the app.
If that does not work try a restart,
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears. Apple recommends you restart the iPad at least once a month.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.