After the calf is made, God tells Moses “go down, because your nation that you brought up from the land of Egypt has become corrupt” (32:7). Moses, trying to convince God not to toast the entire nation, says, “Why should the Eternal’s anger be aroused against your nation that you brought out of the land of Egypt with a mighty hand and with an outstretched arm?” (32:11). After the most severe sinners in the calf incident are either executed by the Levites or fall victim to the plague, God tells Moses, “Go up from here, you and the nation that you brought up from the land of Egypt” (33:1).
Sociologists have pointed out that there’s no such thing as a completely free gift: when you give something to someone else, a relationship is created or maintained. This, I would venture to say, is why both God and Moses refer to the Israelites as “your nation.”
God tells Moses to “go down” because of the actions of “your nation that you brought up,” but then declares His intention to destroy the whole people and make a new nation from Moses alone. If Moses is going to be the only one left standing, why should he go anywhere? Why not stay on Mt. Sinai until the smoke clears? Because God is saying, in effect, “you contributed to making this mess, so you have to participate in cleaning it up.”