I recall hearing something about how in Mussaf of Yom Kippur, Nusach Ashkenaz kneels for each of the three confessions (Cohen Gadol on bull; all Cohanim on bull; everyone on goat). Nusach Sefard adds one when the people hear "Lashem chatas" (declared on the other goat). I don't recall the reason suggested for this. Thoughts please?
Beis Yosef (Orach Chaim 621) cites various sources in the Gemara that say that the people bowed and prostrated themselves each of the ten times that the Kohen Gadol uttered Hashem's Four-Lettered Name (three each for the confessions, and one when designating the goat for Hashem). On this basis, then, he states (following Avudraham) that the paragraph Vehakohanim, describing how they did so, should be said then too, for a total of four times during the Avodah.
So Nusach Sefard follows the Beis Yosef.
Nusach Ari (as arranged by R' Shneur Zalman of Liadi) takes something of a middle ground: it has Vehakohanim four times, but with a notation at the one for "LaHashem chatas" that "here it is not necessary to kneel." The commentary Shaar Hakollel explains that this is based on sources (including the Zohar) that specifically relate the bowing to the word תטהרו, which the Kohen Gadol said only during the confessions.