Matthew Poole and continuators on Leviticus 21:11

11 Neither shall he go in to any dead body, nor defile himself for his father or for his mother.

To wit, into the chamber or house where they ly. This and divers other rites here prescribed were from hence translated by the Heathens into their use, whose Priests were put under the same obligations.

Because upon his fathers death he was actually High-priest, having been consecrated to this office in his Fathers life-time.