Esther 4:16 says לך כנוס את כל היהודים "Go Knas (fine) all the Jews." Why did Esther want Mordechai to go Knas all the Jews?
Achashverosh had sold all the Jews to Haman who was arranging to have them all killed. The Mishna on Kesuvos 40b says "כל מקום שיש קנס אין מכר " "Whenever there is a fine there is no sale." So Esther was trying to fine all the Jews which would invalidate the sale of the Jews to Haman.
The Gemara there explains that this is the opinion of Rebbi Meir. Now that we know that they were going in the opinion of Rebbi Meir we can answer another question. Why did Haman's daughter flip the chamber pot over onto her father?
The Gemara in Chullin 139b says ("המן מן התורה מנין (בראשית ג, יא) המן העץ") that the Etz Hadas (tree of knowledge) is a reference to Haman and Bereishis Rabbah 15:7 says that Rebbi Meir holds the Etz Hadas was wheat (Note that Brachos 40a has a different Girsa). Since this was on Pesach and wheat has the status of Chametz on Pesach, Haman's daughter was fulfilling the Gemara in Pesachim 6a "אמר רב יהודה אמר רב המוצא חמץ בביתו ביום טוב כופה עליו את הכלי" that if you find Chametz on Pesach you have to flip a vessel over it, so Haman's daughter had no choice but to flip the chamber pot over onto her father.
According to the Midrash (have to locate it) Jews attended Achashverush's party when they shouldn't have. Mordechai, Esther's cousin, discouraged Jews from going, but they didn't listen to him.
Esther was, understandably, angry at that, esp. that she was the queen. So, she decided to fine the Jews as a punishment.