Droid Bionic screen won't auto shut off when plugged in

My phones screen will remain backlit and on while plugged in after getting a text or something that would pop up. Even if nothing popped up, if I just unlock it while plugged in, it will never re-lock on its own. This wasn't always the case but I didn't notice it for a while and am not sure what might be causing it. Maybe a background app? Works normally when not plugged in.