The Gemara cites additional homiletic interpretations on the topic of the revelation at Sinai. The Torah says, “And Moses brought forth the people out of the camp to meet God; and they stood at the lowermost part of the mount” (Exodus 19:17). Rabbi Avdimi bar Ḥama bar Ḥasa said: the Jewish people actually stood beneath the mountain, and the verse teaches that the Holy One, Blessed be He, overturned the mountain above the Jews like a tub, and said to them: If you accept the Torah, excellent, and if not, there will be your burial. Rav Aḥa bar Ya’akov said: From here there is a substantial caveat to the obligation to fulfill the Torah. The Jewish people can claim that they were coerced into accepting the Torah, and it is therefore not binding. Rava said: Even so, they again accepted it willingly in the time of Ahasuerus, as it is written: “The Jews ordained, and took upon them, and upon their seed, and upon all such as joined themselves unto them” (Esther 9:27), and he taught: The Jews ordained what they had already taken upon themselves through coercion at Sinai.
In fact Esther - I will surely hide, in the days of Esther there would be hiding [G-d's] face and finding many wicked troubles
Purim and Reacceptance of the Torah, by Rabbi Chaviv Danesh (Ohr Sameach)
When the Jewish People left Egypt they were on a spiritual high. The miracles, and thus G-d’s presence in the world, was so clear that all doubts disappeared. It was this clarity that the Midrash metaphorically refers to as a mountain hanging over the Jewish People. At the time, the revelations were so intense and the significance of the Torah was so apparent that it was almost as if there was no option but to accept the Torah. Therefore, even though their acceptance of the Torah was essentially voluntary, it also had an element of compulsion. On Purim however, when G-d’s presence was hidden, the Jewish People reaccepted the Torah without the coercion of the “mountain” of clarity. This is the reason why it took until Purim to reaccept the Torah. In the midst of G-d’s concealment Purim was indeed the most opportune time for the Jewish People to reaccept the Torah through exercising the full extent of their free will.
Having quoted a verse, the baraita tangentially interprets the subsequent verse. From the phrase: “But with he who stands here with us this day” (Deuteronomy 29:14), I have derived only that those who stood at Mount Sinai were included in this covenant. From where do I derive that the subsequent generations, and the converts who will convert in the future, were also included? The verse states: “And also with he who is not here with us this day” (Deuteronomy 29:14). And I have derived only that the mitzvot that the Jewish people accepted upon themselves at Mount Sinai were included in the oath. From where is it derived that mitzvot that were to be initiated in the future, for example, the reading of the Megilla, the Scroll of Esther, on Purim, were also included? The verse states: “The Jews ordained and took upon themselves…that they would keep these two days” (Esther 9:27), which is homiletically interpreted to mean: They ordained, in the generation of Esther, mitzvot that they had already accepted upon themselves by oath in the plains of Moab.