A source at the Organization for the Prohibition of Chemical Weapons (OPCW) confirmed that laboratory tests had come back positive for the sulfur mustard, after around 35 Kurdish troops were sickened on the battlefield last August. ...
The OPCW already concluded in October that mustard gas was used last year in neighboring Syria. Islamic State has declared a "caliphate" in territory it controls in both Iraq and Syria and does not recognize the frontier.
Perhaps the gas was stolen from Assad stockpiles not actually surrendered as he pledged to do.
Perhaps the gas was made by some of Saddam's boys working with ISIL--remember, Saddam kept the organization and raw materials to restart WMD production. The knowledge base remained and could have been put to use, perhaps with raw materials stolen from Assad's government.
Heck, maybe some of Saddam's boys know where Saddam's old stuff--gas or raw materials--was hidden away--whether in Iraq or Syria perhaps.
The targeting of Kurds speaks of old Saddam Baathists who have a history of gassing Kurds.
On the bright side, the gas might be old, making it less effective considering that Iraqi skills led their gas to deteriorate and required Saddam to have gas manufactured shortly before use (as in months, if I recall correctly) during the Iran-Iraq War.
And it is certainly not available in large amounts which is really necessary to blanket an area with sufficient density of gas to kill. note the Kurds were sickened and not killed.
So this might be just to terrorize and bring up old memories of serious Saddam-era gas killings.
It is also a reminder that the Western revulsion against chemical warfare created from large-scale World War I usage probably isn't a universal revulsion.