Why did Hashem come down alone (in an Amud Anan) and not with an entourage?
Rashi: When Hakadosh-Baruch-Hu goes to war, He goes alone, 1 and when He goes for peace, He travels with His retinue. 2
Rashi: As the Torah writes in the Shirah "Hashem Ish Milchamah".
As the Pasuk writes in Tehilim, 68:18 "Rechev Ribosayim Alfei Shin'an".
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.
Rashi: Like we find by No'ach, whom Hashem referred to "Tzadik" in his presence (See Bereishis 7:1) and as "Tzadik Tamim" not in front of him.