For that he sinned by the dead— i.e. Contracted a legal impurity, by touching or being near a dead body; which, though it was involuntary, yet was a breach of the ceremonial law, and therefore thus to be purged. The reason of this was, according to the most judicious rabbis, to make men more cautious how they contracted any defilement: whence that famous saying among them, Diligence begets caution, and caution purity, and purity holiness.
Related Commentaries of Numbers 6:11
Numbers 6:11
11 And the priest shall offer the one for a sin offering, and the other for a burnt offering, and make an atonement for him, for that he sinned by the dead, and shall hallow his head that same day.