Haman gets stuck taking Mordechai around on the kings horse declaring how Mordechai saved the king. He is forced to show Mordechai greatrespect when he had been expecting it for himself.
Even worse, while leading Mordechai around town, Haman's daughter dumps the days trash on his head, mistaking him for Mordechai.
Haman has had a pretty lousy day.
He gets home and tells his wife and friends what happened to him.
In 6:13 they respond to Haman, "אם מזרע היהודים מרדכי... לא תוכל לו" - If Mordechai is from the Jews.. you will not be able to overcome him."
Why did Zeresh not say somethign like this earlier and try to dissuade him from attacking Mordechai risking these results? What good does it do to tell him this now? She is just rubbing salt on his wounds!!??
This is a classic case of CYA. They are starting to distance themselves from Haman. They see his star has risen and is now on the way down. They are changing teams.
We see a bit further in the megilla in 7:9 when Harvona tells Ahashverosh about the gallows that Haman had prepared a similar event. How did Harvona know about the gallows? He must have been involved with Haman. Yet he very quickly changed sides and snitched out Haman. He saw that Haman was a goner and he acted accordingly.
There is no loyalty among the wicked. Everybody is looking out for their own interests.