In chapter six, Stradlater takes Jane, for whom Holden has a soft spot, out on a date. The whole time they are out, Holden gets himself more and more worked up, thinking that Stradlater would take advantage of Jane. To add insult to injury, when Stradlater gets back, he tells Holden that he didn't like the essay which Holden had written for him.
To make matters worse, Stradlater gets evasive when Holden tries to ask him how the evening went, adding to Holden's impression that something untoward had happened between Stradlater and Jane. At this point, all hell breaks loose in Holden's mind, and he hits Stradlater.
If I had to pick two words from this scene to describe Holden's character, they would be impulsive and jealous. The fact that Stradlater and Jane are out having a good time has clearly gotten under Holden's skin, and the fact that he had a history with Jane doubtless played a part in this. However, he had no right to allow his jealousy to cloud his responses to Stradlater.
Holden shows off his impulsiveness in a negative way when he starts a fight with Stradlater, simultaneously taking a swing at him and verbally assaulting him. This was a complete overreaction to assumptions that he was making about Jane and Stradlater's date.
As the old saying goes, jealously makes you nasty, and it is apparent that Holden did not approve of Jane—for whom he clearly has some residual feelings—spending time with Stradlater.