Most Ashkenazi congregations have the practice of standing up during the reading of the Aseret HaDibrot (Ten Commandments). Where does this Minhag come from?
The Gemara (Berachot 12b) explains why we don’t say the Aseret HaDibrot everyday. There was concern that this might give credence to the position of the Minim (heretics; an uncensored version of Rashi explains that the reference is to the students of Yeishu) that the Aseret HaDibrot are the only divinely revealed Torah commandments. Rashi explains that the Minim will use this to convince unlearned Jews that the rest of the Torah was not given by Hashem.
The Beit Yosef states that although we don’t do read the Aseret HaDibrot everyday BeTzibbur due to this concern, BeYechidut it would be permissible to read them as a remembrance of Matan Torah. In the Teshuvot of the Rambam (number 46 in the Freiman edition and 263 in the Blau edition), he mentions that for the same reason, we shouldn’t stand during the Keriat HaTorah of the Aseret HaDibrot, as doing so also will give the impression that the Aseret HaDibrot are the only divinely revealed portion of the Torah.
Rav Ovadia Yosef (Teshuvot Yechaveh Daat 6:8), after quoting the Rambam and more, rules that the practice to stand is unacceptable and should not be followed. This is the reason that Sephardic shuls do not have such a practice. For Ashkenazim, though, what is the justification?
Gaon Rabi Shmuel says that this practice is especially important. He explains that by standing, we demonstrate the same devotion that Bnei Yisrael showed at Matan Torah, as the Pasuk states, “VaYityatzevu BeTachtit HaHar” “And they stood at the feet of the mountain” (Shemot 19:17). Additionally, it is like we are accepting the Shechinah of Hashem the same way Bnei Yisrael did at Matan Torah, reinforcing their actions with our own. He adds that the problem with the Minim doesn’t apply unless there is no clear reason to stand. Since everyone knows why we are standing during this Keriah, it is acceptable. He further rules that it is totally acceptable only on Shavuot (when we recreate the experience of Matan Torah), and if the congregation wants to, it can abolish the Minhag of standing during the readings of the Aseret HaDibrot that appear in Parashat Yitro and Parashat VaEtchanan (which simply are part of the weekly Torah reading).
This discussion demonstrates the importance of investigating the reasons for Minhagim. If there were no purpose for this Minhag, we would be in violation of some serious prohibitions, as the Rambam noted. Only by delving into the basis for Minhagim can we evaluate whether a Minhag is correct or not.