Why did Hashem come down alone (in an Amud Anan) and not with an entourage?
Why did Hashem call Aharon and Miriam?
Rashi: In order that they leave the Azarah where they had gathered together with Moshe.
Why did He want to separate them from Moshe?
Rashi #1: Because He wanted to relate the full praise of Moshe, and one only relate a Tzadik's partial praise in his presence. 1
Rashi #2: So that he should not overhear when Hashem rebuked Aharon.