What is the significance of this punishment?
Ramban: Refer to 32:27:5:3 & 4.
Targum Yonasan: Refer to 32:20:1:3.
See Sifsei Chachamim here.
Why did it ocur here, after Hashem had alrready punished the chief sinners and instructed Moshe to lead the people to Eretz Yisrael?
Ramban #1: It was an addional layer of atonement to render them worthy of achieving that end.
Ramban #2: The decree had been issued already before but took effect only now.
Ramban #2: The plague was not one that took immediate effect; It took the form of a decree that they would all die prematurely. 1
Ramban #2: The Torah does not always inform us as to the number of people who died. 2
Why does the Torah add the words "asher Asah Aharon"?
Ramban: Bearing in mind that Aharon actually made the Eigel, and the Torah ascribes the plague to "those who made the Eigel", it had to point out that Aharon made it upon their instructons.