Over rated in tourney?

I am playing a player rated 1833 in a tourney that is <1600.  How can this happen?

If you haven't played many games here your rating tends to be very volatile and jump up and down like a trampolinist.  If he's been steadily winning his previous games in the tournament then his rating has simply jumped up.  You could try looking up "Glicko RD" if you want a little more scientific explanation.  Glicko is the name of the rating system chess.com uses.

How long has the tourney been waiting to start? He could've entered at <1600 and won a few games during the wait...

I found the game, that does look like a mistake. He's played 1300+ games and has been rated 1850-1950 for well over a year, never dropping below 1800. Try sending a ticket in, ask them to check it out.