i think he was unhappy with the roles he was getting offered and maybe was kicking himself for passing up on Gandalf. still, his body of work was iconic already, so retiring at that point in his 70s was a good decision. when Jack Nicholson passes on, it will be almost the same way.
i think they did a "how did this get made" podcast on "League" and apparently everyone got treated like shit on that production. lots of stuff left on the cutting room floor so the movie was missing some key elements.He got fed up with the game after being treated poorly on the set of League of Extraordinary Gentleman.