Together, we are the United Nations
Gunulfr ok Øgotr ok Aslakr ok Rolfr resþu sten þænsi æftir Ful, felaga sin, ær warþ ... døþr, þa kunungar barþusk.
what is the reasoning behind the tribes having the special ability of ignoring the withdrawal order?
Well, it's not just tribes - it can also be used stacked with e.g. Bulgarian infantry. I would hazard a guess that the effect is a simulation of superior Turkish defensive tactics, denying the Allies the opportunity to hurt their opponent, while allowing their friends to take advantage of the Allies' confusion.