Why was the Chatas (whose blood is sprinkled on the outer Mizbe'ach) burned outside the camp and not eaten like other Chata'os Chitzoniyos?
Ramban (on Pasuk 3): Because it was brought for the same reason as the Par on Yom Kipur, 1 it followed the same procedure as it.
How did Aharon know to burn the Chatas outside, seeing as Moshe did not instruct him to do so?
Ramban (3), Moshav Zekenim (2): He understood that it is like Chatas Yom Kipur.
Rashi: Moshe did instruct Aharon to burn it outside the camp, even though the Torah does record it. 1
Ramban (3), Moshav Zekenim (2): Since Aharon only did what Moshe instructed him, and Moshe told Aharon only what Hashem commanded him.
QUESTIONS ON RASHI
Rashi writes that the only Chata'os Chitzoniyos that were burned were this (of Aharon) and the Milu'im. Also the second bull of the Leviyim (Bamidbar 8:8), the Chata'os in the days of Ezra (Ezra 8:35) and in the days of Mashi'ach (Yechezkel 45:22, 25) were or will be burned!
Riva (Shemos 29:14): Also of the Leviyim was Milu'im (to inaugurate them), and those in Ezra and Yechezkel. In Horiyos (6a), we say that it was a Hora'as Sha'ah. Rashi means 'except for this, and of the Milu'im, and similar matters.'