Why was Abbey written off Ray Donovan?

Does anyone have the 411 on this?  Why did Paula Malcomson leave the show?  Was there some friction behind the scenes or something?  It's just really odd to write off a central character like this and in my opinion, doing so was a mistake.  Abbey's interaction with Ray was one of the biggest draws of the show.  She was a good foil for him, a mirror, she helped round him out as a person, made him human and much more interesting a character.  I'm just not feeling this change they hit us with?  Anyone have the backstory?