The animosity underlying it is traceable entirely to the Iranian government. It has increasingly pressured the al-Maliki government to dispose of these people who are, in essence, a bone in their throat. They've been there for a while. They call attention to themselves. They call attention to the depredations of the Iranian government.
The Iranian government has quite openly pressured the Iraqi government. Various Iraqi officials have travelled to Tehran. There are communications between the Iraqi government and the Iranians that have been documented, indicating that the Iranian government wants the presence of these people ended. That's really the source of the problem.