The government accused Darfur rebels of kidnapping and robbing foreign aid workers. The aid workers, and especially their convoys of food (and other) aid, are tempting to all the armed groups in Darfur (rebels, bandits, government militias, even the army). There are plenty of usual suspects.
As expected, Sudan ignored a UN deadline for disbanding the pro-government Arab militias. The government knows that the UN can't do anything about it, yet is compelled to feed and care for the non-Arab Sudanese driven off their land.