The usual tip-stall occurs when a plane is flying close to the stall angle... and a turn is commanded with the ailerons...
The down-going surface -increases- the angle of attack for the wing section ahead of the aileron.
As the wind is already close to the stall angle, the effect of the downward deflection -lowers- the angle of attack for stall.
The wing portion then stalls.
This effect is seen almost every day with weight lifters, which commonly tip-stall and roll over after takeoff when beginning the turn to downwind.
With a right turn, the plane rolls left... and the pilot is usually caught by surprise, and keeps the right roll command in, which prevents the left wing from recovering.
The cure for these planes is to not use down aileron! 100% differential, so the aileron only goes up.
For these planes that's all the roll control needed.
For the usual plane, typically a scale model which also is typically overweight for the wing area, tip-stalls occur because the airplane is turned too tightly, or slowed too much. Correcting the flight path with aileron creates the tip-stall.. Caution in steep banks and keeping the speed up is the best cure.