There's a very interesting story over at http://www.genreality.net/breaking-up-is-hard-to-do-choosing-to-leave-a-publisher
by Carrie Vaughn about how this author dumped her publisher, Grand Central, to move to Tor with her dark urban fantasy novels.
Basically, it was all over a non-competition clause.
This type of move normally happens in the shadows as no author wants to bad mouth their old publisher in case they get seen as a troublemaker - not that Carrie does this here, she sounds very reasoned.
Carrie is a bestseller, of course, and has more pull than most, so was able to cut a better deal with Tor (Tor are a bit of a factory... throw books out, sees what sticks, so this might not suit all authors).