this falls under the OS needing to force close/aggressively manage your memory.
do you have live wallpapers? or a lot of widgets/apps/shortcuts on your screen? do you have a lot of multiple screens? all of this takes up memory and the OS is closing out things that it needs to in the background to conserve memory for running/in focus apps.
there is a tab in settings called "storage". you can click in there and free up phone and storage memory. I would suggest you do this and make sure you have as many files, etc as you can on your external SD card. also please smoke over this thread for other discussions.