Poetic justice as a sign of Divine intervention
I would venture to say that the nahapoch hu -- the turnabout -- that is marked points to the poetic justice that was recognized as the signature style of the Divine. Yithro told Moshe that he recognized the G-d of Yisrael as the Deity because He caused what was planned by the enemies to boomerang on them. The words of the verse are "ata yadati ki gadol Hashem mikol haelohim ki vadavar asher zadu aleyhem" (Exodus 18: 11) Some of the words here are implicit. Rashi explains the meaning to be as the Targum says, that the Egyptians plotted to destroy them with water and so were destroyed with water themselves. The Sforno learns differently in terms of what shows the poetic justice. He stresses the plague of the first born, pointing out that it was the parallel to the Egyptian decree to drown all the baby boys. Still, though, the essence of the matter is that the enemy's own weapons of mass destruction were turned against them in an ironic twist of justice.
In the Purim story, there are several turnabouts that indicate the same type of poetic justice. Haman, as we know, was hung from the very gallows that he erected for Mordechai. But there's more! The ring that was given to Haman was transferred to Mordechai. Earlier on, the parade through the streets on the royal horse in royal robes that Haman wanted for himself was given to Mordechai. Even Zeresh, Haman's wife, and most shrewd advisor, did a 180 degree turn on Mordechai. While initially she advised Haman how to plot his revenge on him, she then turned around and told him that he will not be able to overcome this person "mizera haYehudim" but rather will fall before him. And, of course, we have the turnabout of granting the Jews what had been granted their enemies. Though the orignal decree could not be repeated, the Jews were allowed that day of the 13th of Adar (and the additional day of the 14th of Adar in Shushan) the right to kill their enemies -- including every man, woman, and child -- and to take their possessions for themselves. The Jews' triumph shows that the plot against them was turned on its head, and no one but Hashem himself, could engineer such a feat of poetic justice.
The last point is the same juxtaposition between this type of recognition on the part of the Jewish people that we see in Yithro and kabbalos haTorah . Kiyemu vekiblu -- the Jews accepted the Torah once again, willingly embracing the Torah sheba'al peh - the oral learning. Esther is the end of the period of Tanach and beginning of the flowering of Torah sheba'al peh.