So my theory on the sudden early change in CEO is based on the Coronavirus. Iger said he told the board late last year that he wanted to step down and the board started looking for a replacement. I think they had done their search and decided that Chapek would be the next CEO.
Now the coronavirus comes along and that is just bad news for the division that Chapek runs.... potential park closings, cruise ship bookings canceling, and people generally just not going out and buying merchandise. Chapek was probably going to have to make some hard and unpopular decisions in the near future as head of that division. Even if he makes the right decisions, he would still probably end up looking bad. So to protect their next heir apparent, they promoted him to CEO. Now someone else can make those hard and unpopular decisions and more importantly they will get blamed for any missteps.
I think Chapek probably would have been promoted later this year at some point, but the timeline was probably moved up and they made it effective immediately because of the coronavirus.