Turkey has bombed northern Iraq every once in a while for decades.
This is different.

Turkish warplanes carried out a series of coordinated attacks against US-allied Kurdish forces in both northern Iraq and northeastern Syria today, killing more than 70 people, including at least 18 troops loyal to the Kurdish YPG, and pounding the area around the Iraqi city of Sinjar.
Attacks within Syrian territory targeted the city of Derik, in Hassakeh Province, targeting Kurdish military posts. The US arms and heavily backs the YPG, something Turkey has long complained about. The strikes, reflect long-standing Turkish threats to move against the Syrian Kurds.
But it’s not just Syria. The strikes around Iraq’s Sinjar were after PKK fighters in the area, with Turkey’s President Erdogan insisting that he will never allow Sinjar to become a PKK base of operation. The PKK established a presence in Sinjar during the expulsion of ISIS, and have been largely welcomed by the local Yazidi population.

The State Department condemned the attacks, as did the Syrian and Iraqi governments.
The Syrian Kurds have asked the coalition to impose a no-fly zone to protect them from Turkey. They have also suspended their offensive against ISIS in Raqqa in order to defend themselves from Turkey.
There are reports of clashes on the Syria-Turkey border.
Meanwhile, Turkey has threatened to invade Iraq in order to expel the PKK from Sinjar.