Stradlater yawns when asking Holden to do him a favour by writing his English composition for him. Holden is doubly annoyed at this. In the first place, he resents Stradlater asking favours of him. According to Holden, Stradlater is always asking favours of other people out of sheer egotism and self-conceit:
You take a very handsome guy, or a guy that thinks he's a real hotshot, and they're always asking you to do them a big favour. Just because they're crazy about themself, they think you're crazy about them too, and that you're just dying to do them a favour. It's sort of funny, in a way. (chapter 4)
As is clear from this quote, Holden is dryly amused at Stradlater's self-centred behaviour. But he is also irritated by Stradlater yawning right in the middle of making his request; it gives him 'a royal pain in the ass' (chapter 4). Holden is irked that Stradlater can't even be bothered to sound interested when asking 'a big favour' of someone else.