No spoilers about why Pevney chose to leave, though he was never shy about discussing the reasons. But if you look at Trek's four most frequent directors, the third season is clearly a new world. Pevney directed no third season episodes. Vince McEveety and Marc Daniels directed one episode each, and Ralph Senensky directed one episode and was fired during his second. Not to say there weren't good directors in the third year; there were. But it is another reason that the look and feel of the show changed that year.