I'm not sure he decided to resign. "X has decided to step down" is often PR speak for "X was forced to step down".

Both of these scandals individually are usually pretty rough to weather, together they were essentially guaranteed to have him forced out by the board. He's choosing to resign the same way a cop under investigation resigns to avoid more public consequences and to hope to sneak away to a new job.

He is at retirement age and a zillionaire. So he is going to start cashing as he is no longer required to report the sale of the stock. He is going to liquidate.