Usually it's mutually agreed upon when there's a conflict. For example Disney CEO Bob Iger was on Apple's board up until Apple announced they were creating a st

Usually it's mutually agreed upon when there's a conflict. For example Disney CEO Bob Iger was on Apple's board up until Apple announced they were creating a streaming service. Disney was planning a streaming service that would compete with Apple TV+. Iger announced his resignation from Apple's board on September 10.