For his disobedience or carelessness. For though the matter might seem small in itself, yet it was an error in God's worship, wherein God is more severe than in other things; and it was an error of the high priest, who had more knowledge of God's mind herein, and was obliged to more care and diligence, not only for himself, but for the influences of his bad example upon the people.
Related Commentaries of Exodus 28:35
Exodus 28:35
35 And it shall be upon Aaron to minister: and his sound shall be heard when he goeth in unto the holy place before the LORD, and when he cometh out, that he die not.