Upon the heads of the bullocks; to signify, that they were offered by them and for them. See Exodus 29:15,19 Le 1:4 3:2 4:4.
Related Commentaries of Numbers 8:12
Numbers 8:12
12 And the Levites shall lay their hands upon the heads of the bullocks: and thou shalt offer the one for a sin offering, and the other for a burnt offering, unto the LORD, to make an atonement for the Levites.