The Gemara (Megillah 13a, bottom), cited by Rashi to the verse in Esther, says that this was Achashverosh's last-ditch attempt to get Esther to reveal her origins, since otherwise she might be replaced as queen.
(It says that this was done at Mordechai's advice; thus the juxtaposition that "Mordechai was sitting at the king's gate." Me'am Loez adds - I don't recall his source - that Mordechai actually had a reason of his own for this suggestion: to see whether indeed Esther's selection as queen had been ordained by Hashem - in which case Achashverosh would end up finding no one better - or not.)
R. Eliezer Ashkenazi (Yosef Lekach), on the other hand, explains the other way around: now that Achashverosh had found his queen, he was ready to send all of the other candidates home. Since they were in different places in the palace complex or in the kingdom - some were with the agents who had been sent to look for them, others were already under the control of the officials mentioned in 2:3 (ויפקד המלך פקידים), still others in the first or second harem - this can be described as "rounding them up." (The point, R. Ashkenazi goes on to say, is that even now that she was confirmed as queen and there was no reason for her to fear reprisals against her family or nation for her having tried to evade being chosen, she still continued to obey Mordechai's orders and keep her secret.)
Maharal (Ohr Chodosh) seems to say that indeed it was a regular occurrence: the women were brought to the harem in groups (and then each one had her night with the king), and this was the arrival of the next group after Esther had been crowned as queen; Achashverosh might well have selected some of them as concubines. (Maharal therefore explains the relevance of the statement that "Mordechai was sitting at the king's gate" almost exactly the opposite of the Gemara's explanation: he wanted to keep encouraging her not to be afraid that she'd be replaced, but to keep her secret.)
R. Yaakov of Lissa (Megillas Sesarim) explains that Achashverosh didn't want to be intimate with Esther. First of all, since she wasn't telling her lineage, any children he'd have with her might be of ignoble origin; second, if she became pregnant she might lose her beautiful looks. So he had more girls brought to become his wives and concubines, but kept Esther as queen because of her beauty; she liked this arrangement - not being forced to have relations with Achashverosh - and therefore kept from him the information about her lineage that might have made him desire her more.