The Piper Seneca II has rectangular wings. Rectangular wings create strong wing tip vortices. The wing tip vortices have the effect of reducing the effective angle of attack at the wing tip, thust delaying tip stall. Therefore as the wing approaches the critical angle of attack, and given that the wing tip will always have a reduced effective angle of attack (which is what really creates the induced drag), it results that the wing root will stall first.
Let someone confirm this. This is the way I understand it though.