Here's why:
* The "luck" is too good to be true: The gamblers are winning at an unbelievable rate. This doesn't seem natural, and it raises suspicion in Percy's mind.
* The people are different: The gamblers don't seem like regular people. They're all incredibly wealthy and they have a strange, almost hypnotic energy about them.
* The casino is too lavish: The casino is opulent beyond belief, with lavish decorations and a constant sense of excess. This further fuels Percy's suspicions that something isn't quite right.
While Percy doesn't fully grasp the true nature of the casino (that it's run by monsters and that it's part of a larger conspiracy), his intuition tells him that something sinister is happening. This suspicion is what drives him to investigate further and ultimately uncover the truth.