Two other points come to mind now:
The 'Bain & Co' faction in QF management continues its ascendancy with these latest moves. The new 'strategy' department has nearly 1000 staff now. Almost of them ex Bain.
Buchanan is from their arch enemy - Boston Consulting Group, so its likely that this played a significant role in his removal.
Additionally, its likely that there is a couple of 'known unknowns' in this move as well. That is, something Bruce has said or done has significantly upset Eileen and the board.
What that 'thing' is, we will never know, but its of interest never the less.