Is your opponent a diamond premium member? One of the little known benefits is the ability to move your opponent's piece for him once each game. Almost everyone knows about the big orange CHEAT button that makes the best move for you (although only once per game) but for some reason the ability to move your opponent's piece is not so well known.
Sorry Keith, couldn't resist. I have the self control of an 8 year old. 
Obviously I have no clue, but the easiest explanation comes from this question. Anyone else have access to your computer?
Hi, there are couple interesting things that happened in my account today. For one of my games, I was down to about 4 hours on the clock, then my time wasa upped to 29 hours. This is not a problem. For another one of my games, I need to move 23. Qh6, instead somehow a move was made without my consent , 23. Bd2. So now my opponent took my queen for free?!. What on earth happened here? Is there anyway to find out what happened?