"The students of the Maharil (Rabbi Yaakov Moelin, 1365-1427) offer a chilling story about their teacher in his collection of laws pertaining to Yom Kippur.2 The Maharil was once chazzan in the town of Regensburg during the Yomim Noraim, and decided to insert a piyyut into Mussaf composed by Rabbeinu Ephraim, who happened to be buried in Regensburg. Although the leaders of the town informed him that this was not their practice, he did not listen to them based upon his logic, saying that it would be an honor to Rabbeinu Ephraim to recite the piyyut. A few days later, on Yom Kippur, the Maharil’s daughter died. He understood that this was a punishment for changing the minhag hamakom (local tradition).3 This background provides a remarkable insight into why the Maharil emphasized the importance and centrality of minhagim, and how he became the single most influential and accepted codifier of Ashkenazi practices.
"Writing during the time of the sprouting of the Reform movement, the Chasam Sofer (Rabbi Moshe Sofer, 1762-1839) was a strong advocate of keeping minhagim. He felt that unorthodox practice began by “simply” changing a few minhagim. As such, he called those that change minhagim “violators of the Torah.”4 This remark is based upon Tosfos, Menachos 20b (s.v. Nifsal) — “minhag avoteinu…Torah” — the tradition of our forefathers is considered Torah.
"Rabbi Yosef Dov Soloveitchik explained that this phrase from Tosfos extends beyond the basic obligation to heed to the minhagim of one’s community. It also applies to the requirement to study and understand minhagim and their origins, just as one wrestles to understand each and every word and halacha mandated by the Torah to the best of one’s ability. It is based on this that the Rav dedicated much time from his shiurim teaching minhagim and their sources, with the same depth of methodology and rigor that he would use when teaching halachic concepts to his students."
from Minhagim on Chanukah: Dreidel and Sufganiyot, Rabbi Yosef Kalinsky