My guess is these attacks are done by foreign Jihadis, probably Syrians. The Sunnis have nothing but the hope of a central government and what they receive from it, yet they imperil it by their actions which will only embolden the independence wishes of the two other ethinc populations in the country which have Iraq's oil, the Kurds and the Shiites.
Imo the Syrian Ba'athists' plan is to drive the Americans out, take over central Iraq, move against the Kurds (for their oil) and deal with the Shiites in the south as best they can.
The Iranians in the south have a different plan, that is to take over the area, deal with either the Coalition or the Sunnis in the north as best they can and administer their area as a Shiite provence of Iran. In short, Syria wants the norther oil fields and Iran wants the southern oilfields. The 'Sunni triangle' is the Syrians' entry point and base of operations so they want the coalition out to increase their control. They have a long way to go and the 1st MEF to deal with. The Shiites otoh are wary of the Syrians, Ba'athists and the Sunnis altogether so for the time being, prefer the Coalition's presence.
Just my impression.
If the Iranians really wanted to, they could already have made things really hot for the coalition in the Shiite areas of southern Iraq. The fact that they haven't done so already suggests to me that the Iranian mullahs are hanging on by their fingernails inside Iran itself. They simply lack the resources to oppose the US at this time.
For now, the Iranians seem content to watch the events unfolding in Iraq while 'nudging' the local Ayatolla now & then. My 2 cents.