The Malbim explains Haman’s advisers’ words as implying that since Haman started falling already in his degrading display of honor to Mordechai, the natural inertia will cause him to continue to fall.
The Talmud (Megillah 16a) interprets the verse’s double language of “nafol tipol” (“falling you will fall”) by noting that the Jewish people are compared both to the dust, and to stars. The Maharal asks on this Talmudic statement that, historically, every nation has ups and downs. When Babylonia, Greece, Rome, or any other nation sinned, H-Shem caused their civilizations to decline. He answers that the difference for the Jews is that H-Shem directly supervises their ascents and downfalls. Therefore the practical difference is how far up or down.
The Midrash (Bireishis Rabba 8) on Bireishis (1:28) says that man can fall to the lowest depths. There is no middle ground for the Jews; they either fall into the dust or rise to the stars.
R’ Yehonason Eibshutz quotes the Talmud (Brachos 4b) that there is a principle that H-Shem saves the Jews when they are at their lowest. H-Shem waits (kaviyachal, as it were) for the individual Jew or the nation to reach the dust, the lowest point. If a person sitting on the floor falls, that person may fall to the floor. But when one is sitting on the floor and falls, there is not place further to fall. It is at that point that H-Shem must raise that person to the stars.
The Ben Ish Chai writes that the advisers pointed out to Haman that he himself said (Esther 6:11) that the honor received by Mordechai “ye’aseh” (“will be”) in the future tense. He thereby unwittingly cursed himself with Mordechai’s future success.
The Ginzei HaMelech quotes the Ta’amei Esther, who quotes the M’nos HaLevi, who in turn quotes R’ Eliezer of Garmiza that the words nafol tipol are written complete, both with vuvs because Mordechai’s six (the gematria of vuv) actions including fasting, walking through the streets, crying, etc (Esther 4:1) parallel the six (the gematria of vuv) forms of suffering endured by Israel (Esther 4:3). Therefore, Nafol Mordechai and tipol the Jews. The Ginzei HaMelech explains by quoting a Midrash (Vayikra Rabba 34:8) that Moshe accepted upon himself the pain meant for others, and Mordechai did the same. Therefore, paining these kinds of leaders pains all of the Jews. By doing that, Haman is pinning himself against the combined power of the entire Jewish nation.
In a more Kabbalistic explanation, the Rema writes that this falling is actually the second fall, as in the days of Adam (Bireishis 3:15), when the Yeitzer HaRa fell the first time.
The Sha’aris Yosef quotes the Nachal Kedumim that Mordechai was a gilgul, reincarnation, of Yaakov, and Haman was a gilgul of Eisav. One of his proofs for this idea is that the verse (Bireishis 32:12) in which Yaakov prays for Heavenly assistance against Eisav, he says, “hatzileini na miyad” (“save me please from the hand”), which have initial letters that spell out Haman. Although Yaakov never physically fought Eisav, he did fight Eisav’s guardian angel. The verses describing this exchange (Bireishis 32:25-30) never explicitly names the victor, which prompts the Midrash (Bireishis Rabba 77:3) to state that it would remain unclear who won except for the fact Eisav’s guardian angel had to go back and forth several times. Just as Eisav’s angel had to go back down, Haman’s advisers here are implying that, being a gilgul, Haman would similarly fall twice before Mordechai.