Question:

What’s the source of the custom not to wear shoes at the Ohel? Is it just a Chabad thing?

Response:

Precedent for this concept is found in G‑d’s command to Moses at the burning bush, “Take off your sandals, for the place where you are standing is holy ground.”1

Joshua was told something similar when he encountered an angel.2 From these incident, says the Midrash, our ancestors learned not to wear shoes in a holy place.3

The burial place of a holy person is considered sacred ground. We see this in the words of Rabbi Yaakov Moellin (known as Maharil, 1365–1427), the source for many of Ashkenazic Jewry’s customs. Discussing visiting cemeteries during a fast days, he writes that the presence of holy people “makes it a holy and pure place, and our prayers are better received when we pray on sacred ground.”4

Besides for Chabad, this was also the custom of Rabbi Yisrael of Sadigur, the Chernobyl Chassidic dynasty and others.

The Leather Factor

Now in the Temple in Jerusalem, the ultimate sacred spot, visitors were allowed to wear non-leather footwear. The same allowance applies when visiting the graves of the righteous.5

Why?

There are four strata of existence in the world: inanimate objects, plant life, animal life, and humanity. Normally, we must be cognizant of our place on top of this structure, with the other three groups functioning to help us in our divine service.6

This is symbolized by putting on shoes, made of leather (from animals). The message to man is, "You are on top of everything, and you are meant to utilize all of creation to serve G‑d."

(This is reflected in the blessing we say when putting on shoes in the morning: "Blessed are You . . . who has created for me all my needs.")

This applies under normal circumstances. However, when we are in a holy place, we must remove our shoes. In the presence of the Divine, there is a higher force, and man must clearly recognize that he or she is no longer “in charge.”

And when we humbly turn to G‑d in prayer, He will surely hear our prayers, as the verse states, “When My people . . . humble themselves and pray . . . I shall hear from heaven and forgive their sin and heal their land.”7