It all began with a plea to Sistani from a group of well known and respected scholars. They argued that there needs to be congregational prayers held at the shrine of Imam Ali and that it was a shame no one has been leading the prayers. Sistani listened carefully took it on himself to make sure a short-list of men was carefully studied in order to pick a single scholar to lead the prayers. Eventually they decided on Mohammed Taqi al-Hakim, the younger brother of the other Grand Ayatollah Mohammed Saeed al-Hakim.
On thursday last week he led the prayers at the shrine courtyard which was packed and people had to line up on the streets to pray. The Sadrists saw this and were incensed. A scholar from a rival 'clan' was attracting too much attention on their doorstep (Moqtada's office is only several yards away from the door of the shrine) and they were not going to stay quiet.
They let it go the first day but the next day when the elderly Mohammed Taqi al-Hakim approached the shrine with his modest security detail (I have bumped into him several times in Najaf and I see one, and sometimes two, armed men following several paces behind him) the Sadrists in typical mob fashion blocked his access to the shrine and ordered him to go away.
Next a message was sent to the office of Sistani, which is also very close to the shrine. The words left no one in doubt of just how angry the Sadrists were. "If he comes again we will shed blood [that will flood] up to the knees".
Mohammed Ridha al-Sistani, the Grand Ayatollah's son, and most probably on the orders of Sistani himself, told Mohammed Taqi al-Hakim to stop praying at the shrine to avert a potential catastrophe.
The Sadrist mob effectively vetoed the highest office in the Shia clerical heirachy and forced Sistani to cave in under their pressure. Many moderate scholars in Najaf are furious that Sistani has appeased the Sadrists in such an open way and rightly argue the Sadrists will just get bolder and bolder until they claim the entire city when it will be too late to do anything about it.
Moqtada has just slapped the marja'iya in Najaf and from what it seems, the marja'iya has turned the other cheek.