The pit guy is looking through the cards played to see if many small cards have been played which would correlate to +EV advantage and the larger bets the player is now making.
But the funny thing is when they do that they already suspect/know. So it is almost for show. he wants to see if you have a reaction like leaving.
But regardless if it is for show or he is actually looking at the cards it is a warning shot. Often the first warning. Part of the game within the game at most places.