I'm pretty sure it's due to something far less sinister. It's probably just a lack of organization and follow-through...in other words...managerial incompetence...like almost everything in the Arab world, and other third-world countries.
What we have is a collection of Saddam's enemies. No doubt he and his Baathist friends liked to stop by and check out the list, and the coffins, and make sure those people were dead.
It makes no sense in an otherwise Moslem dominated country that the dead were not buried within a couple of days.
There is a chance that the place was being "cleaned up" - hence the use of plastic bags.