sandal has a good explanation. I will just add that it nearly always occurs when there is a yaw introduced at the moment of stall. Use of ailerons in slow flight without coordinating rudder will cause one wing to drop suddenly due to the adverse yaw introduced by aileron deflection. Also flying with rudder only near the stall increases chance of "tip stall". To reduce the tendency to drop a wing, especially on trainers, the designer sometimes specifies a small amount of "wash out" which is a twist of the wing so that the root portion stalls first. This isn't desirable on a stunt ship as it makes inverted flight more difficult and interferes with good snap maneuvers. Usually tip stall isn't a problem with a rectangular wing since it naturally tends to stall from the root out. A sharply tapered wing tends to stall at the tips first making tip stall likely.

This assumes that the wing is built straight and true. A wing with even a slight warp built in will tend to drop a wing at the stall without warning. This is often mistaken for actual tip stall.