It is titled "Why I am stepping down" and then, to the best I can tell, only gives this as a reason: "But the time is right for me to take on different roles".

It is titled "Why I am stepping down" and then, to the best I can tell, only gives this as a reason: "But the time is right for me to take on different roles". In short, corporate-speak for "I'm not going to tell you why, but I'm going to pretend that I am".