The first thing that upsets Holden before the fight is Stradlater's reaction to the essay about Allie's baseball glove which Holden keeps with him. The glove is covered with Allie's poems and is probably the best keepsake That Holden has of his brother.
The other thing that is bothering Holden is the thought of Stradlater being alone with Jane Gallagher. Holden was obviously very fond of Jane and shows some irritation when Stradlater refers to her as "Jean". Holden asks Stradlater "What'd you do?" "Give her the time in Ed Banky's goddam car?" Holden says his voice was "shaking something awful". Holden is starting to unravel because of the rejection of his very personal essay and the thought of Jane Gallagher being treated badly by Stradlater so he tries to punch Stradlater which begins the fight.
The main reason Holden gets into a fight with Stradlater is that he is in love with Jane Gallagher and consequently jealous of Stradlater. Holden understands and appreciates Jane, as shown by the fact that he writes about her many times throughout his book. He knows that Stradlater is too much of an egotist, too much of a sexual predator, to appreciate a nice, sensitive, vulnerable girl like Jane. Holden feels protective and somewhat possessive of her. He knows exactly how Stradlater is going to behave with her when he gets he in the car, and he doesn’t know whether Jane can cope with such an aggressive and experienced young man. Holden’s strong feelings about Jane are revealed by the way he starts a fight with an opponent he can’t possibly beat. Stradlater is rather stupid. He can’t even understand why Holden should be angry at him. He thinks Holden must be a little bit crazy. Nobody can really understand Holden very well, with the possible exceptions of Jane Gallagher and his little sister Phoebe.