Assuming the disagreement angle could it actually be that the disagreement was on the Bluestar? I'm guessing as they will announce the design of the Model X in a month and HAVE shown it to some investors already mostly any important chassis construction decisions must have been locked down?
The Bluestar on the other hand is still very much up in the air. Elon might have gone for steel instead of aluminum and Rawlinson wanted aluminum, or Rawlinson thought aluminum was not possible with intended price tag and hence his expertise is not needed any longer? I don't know of course just speculating wildly, though I guess as a Brit, the long travel might be an important factor. He is the right age to have a frail parent or sibling or some such that he might want to spend more time with - thus personal reasons.