Regarding both Shacharis and Mussaf, I can think of two reasons, assuming that you are in Chutz Laaretz, as in Israel most places will duchan at both.
— the Rheinland (as in Franfurt) custom is to always duchan at Shacharis and Mussaf both. On Yom Kippur, we duchan three times, also at Neilah.
— The Shul planned to duchan at Shacharis but at least one Kohen who wanted to duchan was not available, he therefore requested a Mussaf duchan and as no one was drunk, the Rav permitted it.