Didn't his son post some spoilers online? I think it had something to do with that.
Oh, okay. I sure this has been asked before, but was it creative differences between him and Ridley that led to his departure?
Well, someone named "musicians son" did post some very tame spoilers after being shown a few scenes. I can't see him being fired for that unless he was not supposed to be shown those scenes in the first place or, even less likely, also the source of the set picture leak.
The official reason the composer was sacked was that he wasn't the right fit for the material. Funnily enough, the Wikipedia page for the movie was edited to claim that the composer left because he thought the movie was bad. I assumed that was the son's work and edited it out.