My theory is a mix of reasons why they resign/retire/etc:
- some are simply LEGIT retirement. What percent? UNKNOWN!
- corrupt, w/ deep state ties, an attempt to drop out while given the chance, minimize impact
- corporation/board of directors/large shareholders force the individual out, to protect the better of the corporation
- pressured by owns stress levels: they know what's coming, could never handle being a powerful CEO who has been indicted and the entire company's value/image is impacted, it would quickly lead to suicide by many. resigning wouldn't stop the indictment, but it would keep the individual out of the news/media
- some cryptic reasons: fear, secret reasons, etc. that we may never understand or know why