He's been at the helm for 15 years and he'll be 70 this year. While he made no announcement prior it was thought that he'd be stepping down within the next couple years. There doesn't have to be an ulterior motive, global pandemics, or anything else. He's overseen the largest period of growth at Disney in generations, maybe ever going back to Walt or Roy, Disney owns Fox and his pet project, Disney+, has so far been a major success. He'll be on till 2021 as executive chairman and will certainly ease the transition.