Yes, this is unexpected. Companies of this size like to announce big news well in advance. A change in leadership that is effective immediately seems to imply

Yes, this is unexpected. Companies of this size like to announce big news well in advance. A change in leadership that is effective immediately seems to imply that something sudden happened. That could include Iger being presented with a better opportunity (maybe a pending VP nomination), a sudden change in life priorities (maybe health scare for him or a family member), or an attempt to get out in front of a scandal.