The simple answer: Iran.
As I understand it, a few of days ago U.S. troops started going after elements within Sadr's militia that maintain close ties to Iran. The response was a hail of mortar and rocket fire in Baghdad and Basrah. The attacks killed quite a few people in the Green Zone and elsewhere. At the same time, Sadr, from his hideout in Iran, made a Gandhiesque call for non-violent protest. It seems clear that Sadr and Tehran are playing a game of good-cop-bad-cop/bait-and-switch/pick your metaphor. Gen. Petraeus kept it simple: Iran is shooting at Americans. Unfortunately, some seem to prefer the storyline that Sadr is leading a "peaceful protest," and that the violence is the work of rogue elements within his Mahdi militia.
The willingness to ignore his role in the violence only gives Sadr more street cred by distancing him from the current flare-up, and it allows the Iranians to get away with murder (literally), while sending the message that this type of thing can and will go unpunished. But in the run-up to Gen. Petraeus and Amb. Crocker appearing on the Hill, the Iranians and their proxies will do whatever they can to increase the political pressure in Washington. So far the strategy seems to be working.
