He that gathered the ashes became equally unclean with the others. For the defilement of the people, previously transferred to the heifer, was regarded as concentrated in the ashes.
Related Commentaries of Numbers 19:10
Numbers 19:10
10 And he that gathereth the ashes of the heifer shall wash his clothes, and be unclean until the even: and it shall be unto the children of Israel, and unto the stranger that sojourneth among them, for a statute for ever.