The "tip stall" is actually an uncoordinated stall, meaning you do not have the rudder correction needed for the power setting or flight attitude. This makes the A/C fly in a slip, skid, or forward slipping attitude. When this happens the rearward trailing wing will stall more than the forward leading wing, causing a roll. This is the beginning of a spin and has caused the demise of many full scale and models alike. Staying well above stall will prevent this, as will coordinating the rudder better, however very few modelers have the ability to coordinate that well without the sensory inputs. Most scale A/C need a lot more rudder input at slower speeds, especially with higher power setting due to the fact that they have less tail area to appear more scale, and they tend to be shorter coupled, so P-Factor has a lot more adverse effect.