The most likely reason would be low memory. Closing recently used apps in the multitask bar and rebooting will likely sort things out.
Four finger swipe up the screen to open the multitask bar.
Long press on any app until a minus sign appears on each app.
Press each minus sign in turn to close the app.
Four finger swipe down the screen to close the multitask bar.
Hold Home and Power buttons until the Apple logo appears, then release and allow the iPad to restart.