Albert Barnes Commentary Esther 5:9

"Then went Haman forth that day joyful and glad of heart: but when Haman saw Mordecai in the king`s gate, that he stood not up nor moved for him, he was filled with wrath against Mordecai." — Esther 5:9 (ASV)

He neither stood up nor moved for him—this was undoubtedly a serious breach of Persian etiquette and may well have angered Haman.