Rav Shimshon Rafael Hirsch observes that Moshe first destroyed the Golden Calf, and only then turned to Aharon with the question "what did the people do to you that you brought on them this great sin" (Shemot 32:21). By grinding the calf into dust, Moshe had demonstrated that forceful and determined action for good will triumph. The implied question to Aharon is: why didn't you do the same? What great force did the people bring against you that deprived you of your power to display the same determination? To this, Aharon has no answer.