It could be that he is worried about the stock market implications of Coronavirus and wants to "lock in" the incredible stock price appreciation that took place

It could be that he is worried about the stock market implications of Coronavirus and wants to "lock in" the incredible stock price appreciation that took place during his tenure as CEO. That is often how a CEO's performance is summarized after the fact, and "presided over a tripling of Disney's stock price" sounds better than "a 2.2x" or whatever. In any case, his desire to step down has been widely telegraphed in the press and directly to investors in presentations. He was supposed to retire a few years ago but kept being asked to stay by the board. Now that the Fox deal is done and Disney+ launched successfully, it's as good a time as any to retire.