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!