Nahum M. Sarna discusses the clothing described in Exodus 28:42.
Nahum M. Sarna, Exodus (The JPS Torah Commentary; Philadelphia: Jewish Publication Society, 1991), 185 (logos ed.)
42. Hebrew mikhnasayim, “breeches,” occurs in the Bible solely in connection with the priestly attire. They are here listed separately because they cannot be subsumed under the rubric of vestments that are “for dignity and adornment” (v. 40), and also because, to avoid unseemliness, the priest put these on by himself, unlike the vestments, into which he is helped by others (v. 41).