The end of the year is an entirely arbitrary date. I think they believe that if the date were extended, then it would be extended infinitely. They are being pressured by the Iranians to get those people out of there. So they have decided that's the date. They've drawn the proverbial line in the sand and that has become the date.
There is no outside event that has suggested, or propelled, or in any way recommended that date. It's simply been chosen arbitrarily by the Iraqi government.