Analysts such as Bowman say part of the outrage is driven by the fact that "we would have not defeated ISIS' Caliphate ... without their help" and that the Kurds had just recently lowered their defenses, trusting the US to protect them from Turkey.
Since August, the US has been pushing the Kurds to dismantle their defensive fortifications and pull back their troops along the border that Turkey is now attacking as part of a "security mechanism" framework. The goal was to appease Turkey enough so they wouldn't invade. Kurds agreed to participate "presumably because they trusted the US to restrain the Turks diplomatically," Bowman said.
Note that CNN attributes this versions of events to an analyst, so CNN doesn't fully put their own editorial weight behind the (attributed) statement. So what evidence exists that lends credence to this claim?