Jann is minding his own business. He is playing by himself in the courtyard with a wooden sword: this definitely constitutes minding his own business. Nothing that follows is his fault.
The next morning Milo trips on his blanket getting out of bed and shatters his right hip. This makes him exceptionally cranky; broken legs always do, because he can't walk on them until they're at least mostly healed, but the hip means he can't even get out of bed for the next several days. And he's never broken a hip this badly before, so he doesn't know how long it's actually going to be until he can use it again.
"Tonight I can stop being a rabbit," says Jann. This is good news and should therefore be cheering. "Mother made me eat vegetables for breakfast because she doesn't think I'd better have anything good while I'm a rabbit." This is minor misfortune that has befallen Jann and is not directly Milo's fault, and will probably also be cheering.
"I never said I wouldn't go in them," Milo points out, this being the crucial distinction as far as Milo is concerned. "It wouldn't be that hard to sneak in, I bet. And if I did it right when there was gonna be a new moon, I could just go outside when you were done petting me and nobody might even know."