Thrall appeared to be in a state of WOW TBC Gold reticence for the majority of the fight. He was trying to show Garrosh an important lesson and would like to beat him however, it wasn't going to matter. Then Garrosh made the "this was not your fault" thing and Thrall struck with the elements and killed Garrosh.
I've always maintained that it was because the elements were unable to respond to him immediately afterward was not due to the fact cheap WOW Classic TBC Gold that the guy "cheated," as so many people would like to believe. But that he had been abusive to them because of his anger. He's struggling to come to terms with to this day.