Josiah here is enforcing the policies as to the responsibilities of the priests according to the order established under David and Solomon. That is primarily what he is talking about here. We see Josiah personally supervising this Passover. Again, this is after the time of Manasseh. Josiah did it because the people were so pagan that if he let them do it on their own, they would have very quickly reverted back to what they had always practiced.
Here again is a situation similar to the days of Hezekiah. This was a national emergency, and the king simply assumed control. In his zeal he does it in a way he feels will be acceptable to God under the circumstances. He felt that if these people sacrificed the lambs at their own houses, they would have done it to Baal, and not to God.
