See Malbim's explanation. The gist of what he states is:
Haman had planned to tell Achashverosh about his plan to hang Mordechai. When he arrives at the palace (while he was supposedly sleeping, yet - not what Zeresh advised, as she said to go in the morning),not only does Haman not have a chance to tell Achashverosh of his plans, but he ends up leading his own enemy Mordechai on the royal horse!
Then he goes back home and tells his wife and wise men what happened. So Zeresh says, "Well, if this was G-d's will, apparently, and these people are protected by G-d, this wasn't just coincidence. It was a result of Mordechai's praying and fasting and G-d is protecting the Jewish people. It was not just a strange coincidence that aall this occurred. Furthermore, you started your own fall, and no one else was involved, b/c you went against the advice of your wise men who said to go in the morning, but you went in the middle of the night. Also, you fell before Mordechai did. He was going to fall, had you been able to advise the king. But you fell before Mordechai did."