The "surge" worked, mainly, not because of more troops, but because of more money and weapons. The Sunnis needed money and guns to fight their insurgency. They got those, in part, from Al-Q’aeda in Iraq, but by the time of the surge, AQ had overstepped itself and tried to get control. It started assassinating Sunni leaders and it engaged in very indiscriminate killing, which the Sunnis didn’t like. along with engaging in some violations of the norms of war as the Sunnis saw it.
Into this steps the US and says, "we can give you money and weapons, and all you have to do is take out the AQ people who are trying to get control of you by assassinating your leaders." This then was the "Awakening"—guns and money for dealing with AQ, and for peace afterwards.
Since the endgame in Iraq was about who would control Iraq after the US left, which was indicated by the fact that Iraqi government forces were under heavier attack than the Americans (who were attacked just enough to keep the cost high), the Sunnis said "sure." By accepting the money and arms they got to build up to be in a better position when the Americans left—either for negotiation, or for war.
But the Shia central government is aware of this, and in the past few months, they’ve started arresting and assassinating Sunni leaders, in preparation for when the Americans leave. Remember, the Shia government forces aren’t that impressive. Their last independent major operation was in Basra against the Sadrists, and until American forces intervened and Iranians played diplomat, they were losing.
So, Maliki is trying to get his licks in and weaken or break the Sunnis before the Americans leave.
As a result you’re seeing a spike in attacks because, strangely enough, the Sunni Awakening leaders don’t want to be arrested or killed. You aren’t seeing all-out warfare yet because the Sunnis know the US will step in; the US is helping Maliki with his crackdown, and the Sunnis want to save their forces for the real showdown over who controls Iraq after the US leaves, or perhaps more accurately, who gets how much of the oil revenues.
Iraq is not stable. The problems in Iraq were papered over with money. But now that the money is going away, and Maliki is violating the terms of the American brokered truce, the papered-over problems are re-emerging.
Endnote: In addition to the Awakening, the Sadrists standing down, the completion of ethnic cleansing in Baghdad, Iranian intervention, and the British withdrawal from Basra all contributed. But in terms of the Sunnis, money and guns for peace was the primary consideration.