Albert Barnes Commentary Leviticus 21:10

"And he that is the high priest among his brethren, upon whose head the anointing oil is poured, and that is consecrated to put on the garments, shall not let the hair of his head go loose, nor rend his clothes;" — Leviticus 21:10 (ASV)

It was the distinguishing mark of the anointing of the high priest, that the holy oil was poured upon his head like a crown .

Uncover his head - Rather, let his hair be disheveled. See the note at Leviticus 10:6.