Bob Iger has long voiced his desire to retire, it's been postponed several times. He wrote about it at length in his most recent book. Just pulled the trigger.
I'm not suggesting anything tragic like a fatal illness, a #MeToo event, or a presidential run, but only suggesting that this is an unusually-handled event. Look at his predecessor Michael Eisner, who was forced to resign after losing a boardroom battle -- Iger was announced as the next CEO in March 2005, but Eisner didn't actually leave until the end of September 2005, over six months later.
I also read his book that came out last fall, and he explicitly said that his retirement is on the horizon, implying he was expecting to be in the role for awhile.