Rashi explains that there might have been grumbles about Aaron pushing himself forward and assuming office on his own initiative. Who was he after all? The brother of Moses! Moses was showing bias and giving his brother a coveted job! Nepotism! But the choice of Aaron had been made by God, and no-one had any reason to object.
What about Aaron himself? He might well have protested his own unworthiness. “Who am I to be high priest? I’m no saint. God should pick someone else!” Very humble, very modest – but when God gives you a task to perform you have no right to try to wriggle out. Yes, Aaron, it’s good to be humble, but too much humility is actually a form of pride.
Remember Dickens’ Uriah Heep, who kept saying he was a very ‘umble individual? Boast too much about your humility and it transpires that you are not so humble after all. When a task needs doing you have to gird your loins, to use the Biblical phrase, and get on with it.