A lot of progress has been made in the past few years in trying to get the Iraqi sectarian groups to get along. More and more, the Shiites and Sunnis are grumbling their way towards working for joint development and governance. The passing of the crucial election laws means there will be a flawed--but quite improved--representation in government for Sunnis.
But amid these improvements, a new sort of religious targeting has occurred. Christians, particularly in Mosul, have been attacked and killed, driven out, or have simply fled. Mosul, which was once known for heterogeneity and tolerance, is now swarming with anti-Christian gangs, prompting the government to send 1000 extra security personnel.
There is some speculation as to what has caused this sudden ethnic cleansing. It might be because of Christian protests--local election laws in Niniveh have taken away Christian quotas in parliament, and Christians have taken to the streets on Mosul and Baghdad over it. Such protests may be angering some Muslim minorities, who are looking to consolidate as many seats this coming October as possible (for example, Sunni Kurds). Until elections actually happen, there may be perverse power plays, intimidation, bribery, and all sorts of corruption to try to game the system into giving each group a political advantage.
And therefore! I believe that this is the very reason that provincial security handovers are on a long pause, despite the fact that security is good in a few of these regions (Wasit, Babil, Ta'mim): until the election, American forces want to be as present as is reasonable, and really make sure security is strong enough to prevent intimidation and other forms of disenfranchisement. When the election is over, the crisis will recede--violence against minorities will get Iraqis in more trouble than the payoff they can expect might balance. Therefore, the US is waiting until December to handover.
If Iraq can get this situation under control until the election, the Christians might be safe--but on the other hand, there may be angry reprisals by gangs or extremist sections of political groups that are angry at a raw deal--and looking for a scapegoat. It is easier for extremist Muslims to justify violence against Christians than other Muslims, and probably a bit easier to get away with, given the questionable reliability of some of the Iraqi Security Forces. But if the government can show that it's willing to pull out all the stops to protect its minorities, it will earn a lot of much-needed security credit.