Tip stall

 If you understand what is happening you might be in a better position to avoid it. A wing creates lift by a combination of deflection and low pressure due to the faster travelling air that flows over the top of the wing.
In order for the wing to maintain the low pressure the air needs to cling to the surface. 
As the plane slows it starts to descend so now the wing is not just travelling forwards through the air its also descending meaning the air is effectively coming up from underneath the wing. 

 The more this happens the more it disrupts the smooth flow of air over the top of the wing creating eddy's so low pressure is now replaced with drag, causing the wing to stall as lift is no longer being generated