The reason lies in the events of 5.1. You helped Garrosh establish domination point to strengthen its position on the shores and fight off the Alliance.
How does your "Warchief" reward you? He send you on a special mission with Vol'jin and tries to have you assassinated along with him. THAT basically makes this whole thing personal for you, your character wants payback.
Why Garrosh do this? If its simply to ensure no witnesses why did he send you on the "mission" with Vol'jin in the first place? Perhaps Garrosh fears your power and ability, or your loyalty? You are after all not a Kor'Kron but an Adventurer, a mercenary gun for hire whose loyalties are tied to treasure and loot.
Regardless Garrosh tried to have you killed, and if you want to support someone who covertly tried to have you assassinated then maybe you have been hit over the head too many times by enemies over the years to think straight.