As the plane flys slower and slower, the airflow over the wing changes from totally perpendicular to more spanwise. This happens more at the tip of the wing than at the root. Air spills off the tip of the wing, causing loss of lift at the tip. In a severe situation, the tip will stall from lack of airflow. The wing tip will drop, and the stall will progress inward to the root. A tip stall can be quick and violent, and happens with no warning. It often rolls the plane on its side or even turns it completely over. Since this usually happens when the plane is low and slow, there's little chance for recovery.

Dr.1