Taken from aerodynamics for naval aviators:
Sweepback applied to a wing planform alters the lift distribution similar to decreasing the taper ratio. Also, a predominating influence of the swept planform is the tendency for a strong crossflow of the boundary layer at high lift coefficients. Since the outboard sections of the wing trail the inboard sections, the outboard suction pressures tend to draw the boundary layer toward the tip. The result is a thickened low energy boundary layer at the tips which is easily seperated.
I believe in general that stall patterns are a result of varriations in downwash, and thus induced angles of attack along the span.
J