Rav Yissachar Frand Shlita asks this question here and says that it is a reference to forgoing the honour of our elders / gedolim.
In his answer he writes the following:
The Mir Rosh Yeshiva, zt”l, explains that the sin of issuing a halachic ruling in one’s teacher’s presence is a much more basic sin that merely not showing this teacher the proper respect.
The Talmud relates in tractate Chagiga (14a) that the prophet Isaiah came to the Jewish people before the destruction of the Temple and he gave them 18 curses. He told them of the terrible things that would befall them. Included in these things was that “there would not be found in Israel one versed in Chumash or in Mishneh, in Talmud or in Aggadah; there would be no Judges and no Prophets and no one capable of sitting in a Yeshiva.” But the ultimate curse he told them was that “… they shall behave haughtily, the youth against the elder and the base against the honorable.” [Isaiah 3:5].
We can somehow live with ignorance and with the absence of Prophets, but when does Klal Yisroel descend to the deepest of pits? When do they hit rock bottom? When there is no honor given to elders. The reason for that is because the Medrash says that Israel is compared to a bird. Just as a bird cannot fly without wings, the Jewish people cannot exist without their elders. Elephants can exist without wings, cats can exist without wings, all animals can exist without wings — except a bird.
The rest of the world can exist without their elders. For the Egyptians, the Romans, the French, the Americans, the Italians it is nice to have elders — but it is not crucial to their very being. But the Jewish people is not a Jewish people without their elders. Just as a bird cannot exist without its wings, that which keeps Klal Yisroel afloat is its elders.
Therefore the curse of curses that Isaiah gave to the Jewish people before the Churban was that they will reach the stage that the young people will be disrespectful to the elders.
This said R. Chaim Shmulevitz is the terrible crime of issuing a ruling in front of one’s teacher. When people issue opinions and comment on every aspect of life or Halacha without consulting their elders, they are doing a terrible injustice to all of us. They are stripping us of our Gedolim. That is the crime of Moreh Halacha lifnei Rabo, making a ruling in front of one’s teacher. It is not an affront to the Rebbi per say, it is something that strikes at the heart of what the Jewish people are all about.
We are a nation of tradition. “Ask your father and he will tell you; your elders and they will say to you” [Devorim 32:7]. Without that tradition of consulting the elders, the Talmidei Chachomim, the older generation, we will not exist. Therefore when a student teaches the law in front of his teacher, he is stripping the Jewish people of something that is essential to their whole being. For this reason, there is no forgiving on the teacher’s part of this terrible sin.
And for this reason the joy at the erection of the Tabernacle had to be dampened with the death of Aharon’s two sons — because this lesson had to be learnt. We are not a nation without our elders.
