Under Saddam the Sunnis, a minority, held the reigns of power. The Shiites were kept in check and under foot. After Saddam the Shiites, the majority, became aware of their numbers and began exerting political influence. This led to former regime elements, feeling their power being threatened, to take matters into their own hands and begin a campaign of terror to keep the Shiites "in their place". The Shiites did show great restaint until the carnage became to great, now some are looking for payback and are giving the Sunnis a dose of their own medicine.

The same thing happened in the South after the Civil War. Replace the lynchings with car bombs and kidnappings. It's the extremists on both sides stirring this pot, most Iraqis simply want to live in peace.