When the aircraft stalls, the nose tends to drop and decrease the angle of attack. If the tips of the wing stall before the inboard section, however, one tip will invariably start to drop before the other, and as it drops, its effective angle will increase. The stall depends on the tip, and it continues to fall. The aircraft starts rolling as the opposite wing tip rises.
The stalling of the tip of an airfoil in which the remaining portion of the airfoil remains unstalled. It is an undesirable characteristic and can lead to phenomena such as pitch-up, etc.