They have never really talked about it.
Some magazine says that he broke up becuse he was a little jealous on Robert, and other says that Kristen broke up with him so that she can be with Robert. None of them have talked about it.
And Michael have been seen kissing a other girl a couple days after the break up. And Kristen and Robert wear at the Kings of Leon concert with the rest of the Twilight cast and people says that they wear kissing. And a couple a days ago Kristen and Michael wear spotted together. I dont really know if that is true becuse there is no picture.

I think that both of them wanted to break up.