I am not sure what reason Stern is giving for stepping down, but my guess as to the real reason is two fold. I believe that Stern has become a lightening rod for criticism of his strong arm tactics and that reflects badly on Obama (plus Stern is White). Second, I believe that Stern feels that he has accomplished his goals in eclipsing the AFL unions in the US and is moving on to the International level.
The plan is to create a worldwide SEIU to cover the poor in Africa and South America (and probably Palestine) whose health care and pensions will be supported with US dollars.
I hope someone is keeping an eye out on this guy! I am sure there is something hightly unethical he is up to.