Why does Stradlater hit Holden hard enough to make his nose bleed?

Answer 1

Holden is bombarding Stradlater with questions about Jane because he knows Stradlater is a womanizer. Stradlater refuses to tell Holden what had happened, so Holden hits Stradlater in the face. Stradlater then pins him down to floor and tells him repeatedly to shut up. Holden continues calling him a "goddam moron" until Stradlater is so infuriated that he knocks Holden to the ground hard enough to make his nose bleed.

Answer 2

Stradlater hits Holden because they get into a physical altercation over a girl they both know, causing Holden's nose to bleed. Stradlater becomes angry and loses his temper during the argument, leading to the physical violence.