Adam Clarke on Esther 2:21

Mordecai sat in the kings gate - Mordecai might have been one of the officers of the king, as the gate was the place where such usually attended to await the king's call. It is not likely that he was the porter; had he been only such, Haman could have removed him at once.

Two of the king's chamberlains - Eunuchs. Why they conspired against the life of the king, we are not informed. The Targum says that they found out that Esther had intended to use her influence with the king to get them removed from their office, and Mordecai put in their place; therefore they determined to poison Esther, and slay the king in his bedchamber. It is very likely that they were creatures of Haman, who probably affected the kingdom, and perhaps were employed by him to remove the king, and so make his way open to the throne.