Also, perhaps some Shado-Pan occupied the rest of the Kork'ron to buy Taran Zhu time to attack him, a distraction attack. The Korkron wouldnt defend Garrosh as they know he can handle himself, and the Shado-Pan htink of Taran Zhu as their best warrior, so won't think he needs much support. Zhu had never seen Garrosh in action before, so probably though he could take him, the speech confirms this a little. As for single combat on Garrosh's behalf? He is many things, but he is not a coward, he still has some honour in his mind regarding duels and fighting a worthy foe one on one.
All in all, makes sense to me.