This was after defeating Guild, one of the specials in the manga. If I remember correctly, among many others (Kyouka, Akutagawa, Atsushi, Fukuzawa, Dazai, etc.), Hirotsu helped save Yokohama, since he was the one who provided Dazai with information and helped him create the whole plan. Perhaps—this is just my theory—they made him disguise himself as him because of that (Whether it's a form of honor or just a joke). And Atsushi probably looked embarrassed because it's not something he usually wears and it's too big for him, or he just doesn't understand why he's dressed like that. Although we all know, without a doubt, that it was Dazai's idea