Masquerading on Purim symbolizes the fact that the whole Purim incident was wrapped in "the hidden nature of God's countenance." The Purim events all happened in a seemingly natural manner. God's name isn't even mentioned in the Book of Esther! The very word "Esther" means "hidden," and it appears in the Torah in the Hebrew phrase "I will hide My Face." (Deuteronomy 31:18)
