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,
and when He goes for peace, He travels with His retinue.
As the Torah writes in the Shirah "Hashem Ish Milchamah". (His harsh rebuke of Aharon and Miryam was like war.)
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 relates only part of a Tzadik's praise in his presence.
Rashi #2: So that Moshe should not overhear when Hashem rebuked Aharon.
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.
Perhaps He was concerned only lest Moshe hear the rebuke of Aharon, for in any case he will know that Miryam was punished for a misdeed. (PF)