Jamieson, Fausset and Brown on 2 Chronicles 30:16-18

16-18. the priests sprinkled the blood, which they received of the hand of the Levites--This was a deviation from the established rules and practices in presenting the offerings of the temple. The reason was, that many present on the occasion having not sanctified themselves, the Levites slaughtered the paschal victims (see on 2Ch 35:5) for everyone that was unclean. At other times the heads of families killed the lambs themselves, the priests receiving the blood from their hands and presenting it on the altar. Multitudes of the Israelites, especially from certain tribes (2Ch 30:18), were in this unsanctified state, and yet they ate the passover--an exceptional feature and one opposed to the law (Nu 9:6); but this exception was allowed in answer to Hezekiah's prayer (2Ch 30:18-20).