Jamieson, Fausset and Brown on Esther 2:21

Es 2:21-23. Mordecai, Discovering a Treason, Is Recorded in the Chronicles.

21. In those days ... two of the king's chamberlains ... were wroth and sought to lay hand on the king, &c.--This secret conspiracy against the king's life probably arose out of revenge for the divorce of Vashti, in whose interest, and at whose instigation, these eunuchs may have acted. Through the vigilance of Mordecai, whose fidelity, however, passed unnoticed, the design was frustrated, while the conspirators were condemned to be executed and as the matter was recorded in the court annals, it became the occasion afterwards of Mordecai's preferment to the place of power and influence for which, in furtherance of the national interests of the Jews, divine providence intended him.