Jamieson, Fausset and Brown on Exodus 4:14

14. the anger of the Lord was kindled against Moses--The Divine Being is not subject to ebullitions of passion; but His displeasure was manifested by transferring the honor of the priesthood, which would otherwise have been bestowed on Moses, to Aaron, who was from this time destined to be the head of the house of Levi (1Ch 23:13). Marvellous had been His condescension and patience in dealing with Moses; and now every remaining scruple was removed by the unexpected and welcome intelligence that his brother Aaron was to be his colleague. God knew from the beginning what Moses would do, but He reserves this motive to the last as the strongest to rouse his languid heart, and Moses now fully and cordially complied with the call. If we are surprised at his backwardness amidst all the signs and promises that were given him, we must admire his candor and honesty in recording it.