Why did Maurice hit Holden in "The Catcher in the Rye"?

Maurice is a bully. He likes to intimidate people and likes hitting people if he feels pretty sure they won't hit him back. With Holden he must be sure he won't get involved in a fight. That could cause a noisy scene. It could result in Holden getting seriously injured and having to go to the emergency hospital and involving the police. Maurice knows he is nothing but an ignorant flunky and will never be anything better. He also recognizes Holden as being far above him in intelligence, education, and social status. This is what really infuriates him. Then Holden provokes Maurice into hitting him by telling him off and predicting his future fate.

"You're a dirty moron," I said. "You're a stupid chiseling moron, and in about two years you'll be one of those scraggy guys that come up to you on the street and ask for a dime for coffee. You'll have snot all over your dirty filthy overcoat, and you'll be--"

Maurice actually only hits Holden once.

All I felt was this terrific punch in my stomach.

The five dollars they extort from him means nothing to Holden, who shows that he is careless about money. What troubles him is the principle, the injustice, and the sordidness of the whole business. They are things that Maurice knows nothing and cares nothing about. And it shows the difference in their characters and their social class. Maurice actually is pretty stupid. Holden may be wrong about the future he predicts for him, but the prediction is so vivid that Maurice can actually see himself in that position two years into the future. He has seen plenty of "those scraggy guys" on the streets of lower New York, and he feels that Holden somehow is putting a hex on him with his words. Holden is really brilliant. He has a genius IQ and doesn't realize it. His intelligence causes him a lot of trouble, especially with "morons," of whom there are many, and not just in New York. Meanwhile Sunny is hearing Holden too, and his words may be changing her attitude about her wise and powerful protector, reducing him to a very small size.