It didn't just come back, it seems this portion has restarted entirely, 'kill counter' reset to zero. The clouds rolled back in, and the Argent Dawn tents in each capital poofed back into existence. Was its disappearance a bug with patch 3.0.3? Did the Scourge retreat after a certain number of battles won, and they weren't supposed to? The magic number we've been seeing thrown around is 300 battles, but we don't have anything to back that number up, just anecdotal evidence. A lot of people have said the same thing, but repetition doesn't mean it's true.
Not all servers saw this go down, so what happened? We have no idea, but the invasion is not over. It just sort of nodded off for a little. I guess the Herald of the Lich King needs a nap now and then.