I'm pretty sure that if Scott is stepping down it's his decision. You know Bush never fires anyone without strong reason. Scott has actually been doing that job for quite a while, and PSs don't last that long. Seems like he's been in there longer than Ari was.
Agree that Bush is loyal (sometimes to a fault) and never fires anyone - my guess is that Bolten may have asked Scott to leave. If true, he may have also asked Karl Rove to give up DCOS post and concentrate full-time on stratergy - - something I think would be VERY beneficial to the Pres