After returning from the mountain having left Harun (as) in charge for the 40 day period, Musa (as) finds the people worshipping a calf. This was clearly against what he commanded them with before leaving. He immediately turns to the one he left in charge to inquire about what happened. When someone is in a leadership position, their status does not preclude them from accountability and transparency. The questions Musa (as) asks are also important. Despite grabbing Harun's beard, he does not immediately jump to a conclusion. He asks intelligent questions to gauge what happened while giving Harun (as) a chance to respond.
See the explanation from Sh. Yahya Ibrahim below: