While it's not something new exactly, it seems a lot of people haven't quite seen it happen yet. Along with the Scourge Invasion in places like the Burning Steppes, Azshara, and other points around the world, the invasion has hit some of the capital cities. Every now and then a strike team of Scourge, a Pallid Horror and Flameshockers, will spawn in a capital city and start attacking nearby guards and NPCs. When defeated, they'll leave behind a broken Necrotic Crystal which will have a quest for you.
This is something exclusive to the invasion, but it's not something that just started happening today, and not the next step/phase of the event. A lot of people are under the impression that it is simply because it happens so infrequently. It really is rather cool when it happens, though. The first time I saw it was back in 2006 when this event happened the first time. Since I roleplay when I'm not raiding, I was sitting in the Cathedral of Light in Stormwind just yapping away with some people. The Scourge actually spawn in the 'basement' of the Cathedral, so my first experience with them was seeing them rise up out of the catacombs. Freaky? I think so!