We assume power corrupted them. No. Usually, he (or she) was rotten from the start. He was just under the radar until he got into the visible top job; he didn’t become incompetent but was always incompetent as a business leader. So what happened?
He had a good predecessor and rode those coattails for a while. (That’s why corporate analysts always look at how long a person has been in place when rating a CEO.) He was unable to maintain what the previous CEO established, and it caught up with him when more was needed.
He got rid of good subordinates for some reason, usually personal insecurity or lack of political savvy to keep good people. He couldn’t retain a lot of competent people he depended on and they quit and left; he then hired incompetent people to replace them.
Changes in his life: divorce, illness, and/or money problems. Although everyone has similar problems, he didn’t deal with it well. He got interested in doing other things (sex, drugs, rock ‘n roll).
He lost interest in growing the company — lost his concentration and concern. “I took a 132 year-old company down. I managed to put it out of business. I knew I was making bad decisions. I became incompetent, started not to trust others, and got rid of good subordinates.”
No one was watching; no one was there to say “no” to his greed. He didn’t turn greedy, he was always greedy, and the opportunity presented itself to act. “It was there for the taking and everybody else was doing it.” Although technically talented in something, he had no morale code to withstand rigors of top job requirements over time.
All CEOs are not great leaders (they aren’t all grown-up Eagle Scouts), they are just people who showed talent in some area at some point in time that got them into the top job. Getting there is different from staying there.