Before I open a ticket ingame: Is anyone else experiencing issues with the first wave of mobs that attack the horde camp? It displays "Wave 1 of 8" and "14 enemies alive", although I don't see a single enemy attacking the camp.
After a while the second wave is getting displayed, which does arrive at the camp. Same for wave 3 to 7. After the 7th wave the friendly NPCs including Thrall just disappear and the event has to be restarted, resulting in the same issue that the first wave just doesn't show up anywhere.
Yes, this happens sometimes. Just wait for everything to respawn (it takes a few minutes) and go again. I've never had the second try go wrong so far, so I guess either I'm lucky or you're unlucky; just keep going and it should sort itself out.