I thought it was odd that Jack Wall departed before Mass Effect 3, given that he had composed the soundtrack for the other two games which in part gave them its feel.
Although I believe in an interview he said it was time to part ways, I do wonder whether there is more to it than that. Perhaps Bioware/EA wanted a film composer involved, given the scale of the game, which is why they brought in Clint Mansell. Or perhaps Jack was annoyed that they were doing so and he didn't want to work with him, after having created the soundtrack all himself.
If you look at the ME3 soundtrack, there are actually only a handful of tracks actually done by Mansell, so that's another reason I think that might be the case. Although a couple of them are good, I think ME2 had the best soundtrack of the lot.
