Mordecai probably refused the required prostration, usual though it was, on religious grounds. Hence, his opposition led on to his confession that he was a Jew Esther 3:4.
Related Commentaries of Esther 3:2
Esther 3:2
2 And all the king's servants, that were in the king's gate, bowed, and reverenced Haman: for the king had so commanded concerning him. But Mordecai bowed not, nor did him reverence.