Isn't that frequent in series? He hasn't directed all episodes in Mr Robot either.
I've always found that strange, but I'm not entirely familiar with the exact boundaries between the different roles. I suppose they try to keep some coherence, although I remember some episodes of Spooks for example (MI5 in US) that were very badly filmed on purpose - camera shaking and other ill-chosen "action" effects - with a specific director, while it was fine with others (they must have disagreed on that point).