Albert Barnes Commentary Esther 3:2

"And all the king`s servants, that were in the king`s gate, bowed down, and did reverence to Haman; for the king had so commanded concerning him. But Mordecai bowed not down, nor did him reverence." — Esther 3:2 (ASV)

Mordecai probably refused the required prostration, usual though it was, on religious grounds. Therefore, his opposition led to his confession that he was a Jew (Esther 3:4).