A Hierarchy is a system or organization in which individuals or groups of individuals are ranked one above the other according to status or authority. Most schools of Anarchist thought argue that all hierarchies are unethical and should be abolished altogether.
Well, their Anarchist theories are all heavily rooted in Egalitarianism which means they believe that most (if not all) imbalance of power is oppressive and therefore unethical. As far as my understanding goes I believe the reasoning behind their use of Egalitarian ethics is that Egalitarian philosophy puts forth the stance that Equality is the natural state of society. This implies that "natural" is "ethical" and if Equality is natural then Equality is also ethical. From this train of thought, it can then be deduced that the opposite of Equality is also the opposite of ethical or in other words, "Inequality is unethical". Hierarchies in any form are made of people with differentiating amounts of power in that group so since they don't all have the same amount of power (Inequality) then they are unethical and must be abolished.
I'm not an egalitarian type of anarchist though so if I am wrong than please let me know.