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 the plague occur here, after Hashem had already punished the chief sinners and instructed Moshe to lead the people to Eretz Yisrael?
Ramban #1: It was an additional 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 #1, Moshav Zekenim #1: 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, Moshav Zekenim #2: The Torah does not always inform us as to the number of people who died. 2
Why does it say "Al Asher Asu Es ha'Egel Asher Asah Aharon"?
Ramban, Moshav Zekenim: The plague did not kill those who bowed to it and sacrificed to it, rather, to those who congregated on Aharon and brought gold to him. Aharon made it upon their instructions.