Greetings,
I find that usually happens on large apps in my iPad 1. On other generation iPads it is usually due to running out of operating memory. If you have too many apps still loaded in memory you can be low on memory. You can check how many apps are still loaded and maybe running by double clicking the home button or doing a four finger swipe upward gesture. When the screen view slides upward it will show you the apps that are still in memory. I have seen more than 40 apps hogging up the memory. If you touch and hold one of the apps until they start to jiggle a red circle icon with a minus sign will appear on the upper left corner of the app. Touching the minus sign will close the app but not delete it from the iPad. This will free up memory for other apps to use,
Sorry if you already knew this,but I hope it helps if you try it out.