The reason why they chose 'Buratha'.

"A new massacre has struck Baghdad when three suicide bombers attacked the Buratha mosque in Baghdad this afternoon. "More than 70 people were killed and more than 150 were injured" a doctor from Baghdad's medical city told me in a phone call.

A closer look at the targeted mosque makes me think that the ramifications of this massacre can possibly be much worse than the immediate death and pain this terror attack brought, the Buratha mosque is not an ordinary mosque, it has a special religious value for Shia Iraqis as it's thought to be one of the places where Imam Ali stayed and prayed. But that's not the most important thing because this mosque is of considerable political significance, the preacher in this mosque is Jalal Addin al-Sagheer, a cleric from the SCIRI who was the first SCIRI member to publicly urge Ibrahim al-Jafari to withdraw his nomination for office.
This mosque is one of the headquarters of the SCIRI and its clerical wing in Baghdad, even that Abdul Aziz al-Hakeem's son Ammar al-Hakkem preaches occasionally in this mosque when sheikh Jalal is not available.

A military confrontation between the Sadr militias and the American (and possibly Iraqi) army is imminent and it's the Sadrists themselves who are pushing in this direction and preparing their forces for a battle they want to have to disrupt the political process and drag Iraq into an irreversible state of civil war."
ITM

The only problem I see is that this conflict would spread directly into Iran, Shi'a against Shi'a. But look at the up side, Iran is at the table.