The Rabbis of the Talmud have recorded (Shabbos 88a) the tradition that although the Jewish people accepted the Torah at Mt. Sinai out of their own free will, there was, nevertheless, an aspect of coercion involved. After the miracle of Purim, the people accepted the Torah again, this time without any element of coercion. Today when we observe Purim, one of the themes being celebrated is this second accepting of the Torah. According to the Geonim, this is the reason that Purim alone was singled out from all the other holidays instituted by the Rabbis (and recorded in the Megillas Taanis) to have a mitzva of seuda (eating a festive meal). Just like on Shavuos the Talmud tells us (Pesachim 68b) that all agree that one must have an elaborate meal as part of the commemoration of our accepting the Torah, so to Purim must be celebrated with an elaborate meal for this same reason.
According to the Medrash, the element of coercion at the time of maaamad Har Sinai that necessitated the later second acceptance was regarding the Torah She'beal Peh. The Jews were fully prepared to accept Gods written Torah, since it was clearly of divine origin. But the bulk of the Oral Law consists of laws classified as "divrei sofrim", laws developed by the rabbis over the generations, which have the status of dinim doraisa. The discretion and the judgement of the rabbis is assumed to have been divinely inspired, and therefore has been endowed with doraisa status. The verse in Tehilim (25,14) that G-d reveals his secrets to those who fear him is quoted several times in the Talmud to bring out this point.
This is in no way a contradiction to the principal developed by the rabbis that "lo bashamayim hi" (see Bava Metzia 59b) - that after mattan torah God will no longer reveal any halachos to man in a supernatural fashion, i.e. through prophecy, and any bas kol proclaiming a halachah must be disregarded. Of course God expects us to work out the halacha. At the same time, He has promised to assist the rabbis - from behind the scenes - in their deliberations to see to it that they do not err. The binding force of any psak of any rabbi is based on the assumption that the individual posek was granted this supernatural divine assistance.
Bnei Yisroel at the time of mattan Torah apparently found it hard to accept this concept (see Meerot Neryah p. 16a). At the time of the nes Purim a group of rabbis known as the Anshei Knesses Hagedolah was setting all the forms of religious observance as they are still being observed today, two thousand years later (brachos, tefilos, categories of halacha). The Jews realized that the yad Hashem was involved in the story from behind the scenes. They came to understand well the concept of "sod Hashem leyereiov." It becomes understandable that halchos are developed by the rabbis with the yad Hashem guiding them. This is what the Torah Shebaal Peh was always about.
Now that the Jewish people had accepted that part of the Torah again without any coercion, this segment of halachos was able to flourish and to develop in a much greater fashion than ever before. Indeed, the greatest part of the development of the Torah Shebaal Peh took place, historically, after the days of Purim (see Be'Ikvei Hatzon p.138, 114).
The Shalah, in his essay on Purim, points to the posuk in the megillah (8:14) "vehados nitnah beshushan habirah", as an allusion to the concept that the Torah was being reaccepted. The traditional festive Purim meal is eaten to celebrate this reacceptance and should be eaten with such an attitude. Becoming drunk and rowdy simply does not fit in with the correct attitude which should pervade the Purim observance. Purim is not the Jewish Halloween. The custom of putting on masks, and dressing up to conceal one's true identity was never intended to represent a Jewish Mardi-Gras; but rather to show that just as in the story of Purim, one had to look below the surface to see the hidden Mover behind the events, so too in Torah study, one must always look below the surface, and read in-between the lines to gain the insights of the Torah Shebaal Peh, which will actually place everything in the Torah in the proper perspective. The custom of masquerading is to teach us, "al tistakel bekankan elah bemah sheyesh bo!", (Never look at the outer appearance of the container. Always try to investigate what might possibly be hiding beneath the surface.) The fact that God's name never appears in the megillah is also assumed to be for the same reason. The hidden Torah Shebaal Peh interpretation always enlightens the Torah shebiksa, and always puts things into clearer perspective.