From what's been said publicly by principals in the story, we know that Ohanian had taken the AMA section into his personal portfolio. By the way: that by itself is weird: the chairman doesn't usually have an operational role in the company, among other reasons because it's a conflict of interest. Regardless of who he was, he had ownership of the department Victoria Taylor worked in. In most well-managed companies, that makes hire/fire his prerogative, and coordination of personnel changes his responsibility.
When people screw hire/fire up and cause disruption, the CEO normally fires or demotes them, and gets extra careful about future promotions and delegations to keep there from being a pattern of poor decisions about delegation. Here, neither thing was allowed to happen: Ohanian is effectively unfireable, and Pao wasn't given a chance to not delegate more stuff to him.