The Iyun Ya'akov cites the Maharsha's reason as to why they read 'Ha'azinu' ('ha'Ziv Lach') for Musaf of Shabbos. He suggests that it is because Moshe was the one to suggest a day of rest in Egypt, and he picked Shabbos (as we say in Musaf, 'Yismach Moshe be'Matnas Chelko'). And what's more, he adds that Moshe died on Shabbos (according to some), and when Moshe died, the splendor and glory of Yisrael departed ('Panah Zivah Zivah ... ').

Be'Virchas Kol Tuv

Eliezer Chrysler