Here’s an interesting perspective from the BBC’s John Simpson:

Looking back on the events of the past year, it is clear that the three different popular votes which were held in Iraq, two elections and one referendum, played a big part in whipping up the violence.
People who had tended to regard themselves primarily as Iraqis were suddenly forced to focus on the fact that they belonged to a particular group: Sunni, Shia, Kurdish, Christian or whatever.
The act of voting was as divisive as it was empowering, and the fact that it happened three times in 11 months added to the intensity of the problem.