These Warlocks refused to join with Garrosh unlike others.* The Kor'kron knew that if they couldn't keep these warlocks in their side they had to be put down before they could destroy Orgrimmar themselves. - It was shown with that one Warlock that summoned a Doomguard to fight two Kor'kron. On his own without the interference of the heroes this lone warlock could have defeated even more Kor'kron on his own.
That's how I see it and it's one of the little details that make me love my class even more.
*(Assuming Shadow Mages are Warlocks... Which they were in the WoWRPG books. They were a variant class of warlocks)