The fact that he opposes the agreementand calling for regular protests should come as no surprise. Sadr is the only real Iraqi nationalist amongst the Shia leaders, with Dawa and the ISCI being pretty much in the pocket of Iran.
It is also a position which is likely very popular in Iraq, as all independent assessments show that Iraqis as a whole want the US out, and this agreement is clearly a part of Bush’s “Iraq forever” policy, and would keep troops and bases in Iraq for decades.