Syrian Sunni militia

The operation to save Mosul, Iraq's second biggest city, from the Islamic State of Iraq and the Levant (ISIL) has begun. It is led by the Shiite-dominated Iraqi army, Shiite militias, Kurdish Peshmerga forces, some Sunni forces trained by Turkey, and of course the United States, which is coordinating the whole effort. Turkey, however, is rather angry about what is going on. Why?