Commentaire sur Les Nombres 28:15

ושעיר עזים וגו׳ AND ONE KID OF THE GOATS [FOR A SIN OFFERING] — All goats brought as additional offerings are intended to atone for those sins committed by causing uncleanness to the Sanctuary or its holy things, — all as is set forth in Treatise Shevuot 9a. The goat of the New Moon differs from other goats brought as additional offerings in the fact that the word לה׳ is stated of it: that it is a sin offering to the Lord. The Halachic explanation of this is: it is to teach you that it only makes atonement for uncleanness where there is no consciousness of having caused it either before or after it has been done (cf. Rashi on Leviticus 16:16 and Note thereon), so that no one is cognisant of the sin except the Holy One, blessed be He, alone. (It is a חטאת לה׳, a sin offering for a sin known only to God). And that the same applies to other goats (i. e., that they atone for uncleanness under the same conditions) we derive from the law about this goat. — But its Aggadic explanation is as follows: The Holy One, blessed be He, said, “Bring an atonement offering for My sake, because I diminished the size of the Moon" (cf. Rashi on Genesis 1:16; Shevuot 9a, Chullin 60b).

All the [musaf] goats… Meaning, why do they bring a he-goat on the New Moon as a sin-offering — for what sin [was it brought]?