I haven't been able to figure out why. It's not a phone call or mail or instant message -- it doesn't happen with any regularity, generally a couple times a day -- not related to battery levels -- not notification of app update -- I'm not pressing on any buttons. It will do it when it's on (latest time while playing Sudoku) as well as when it's been in my pocket for hours. Any thoughts on either what it is or what I could do to try to track it down?