I'm sure that the main reason behind his departure is that Steve was unsatisfied with his live drumming. They said (including Bruce), that sometimes he played so slowly that it got boring. And Steve and Nicko really had to play together : the way they speak of each other (or the way Adrian spoke of the pair) really makes me think of the Lee-Peart duo. As a listener, I still prefer Clive's drumming, because it sounds more "Metal", but I know that technically, there is a real gap between the two and that Nicko allowed Steve to write in different directions, just like Bruce did one year before.
EDIT : and I have the feeling that Steve now wants to clear out some of the black clouds of the past. The way he phoned Dennis to invite him to a gig and keeps in touch with him ; him meeting some of his old bands members. I think he is one of those guys who get smoother with age.