Why does my computer keep going back to the desktop?

For the past two days my computer has started returning to the desktop randomly. It happens while I'm browsing webpages, typing or whatever. I'll just be typing along and it'll right swipe itself to the desktop.

I have rebooted and changed the batteries in my mouse (they were getting low). I'm not sure what else to do. It has done that twice while I've been typing this.