They are staff run, and usually it is a shard wide threat/invasion type scenario. The last major one was the "Angry Townspeople." Townies that were uprising to overthrow the Queen. Before that it was Branubians, which were shabooli from the future (I think. I started at WOD in the beginning of that one.)
They are normally extreamely fun and well thought out, though every now and then it can just tick you off. Even then it can still be fun.
Lately it's been a lot of little things. I think they are gearing up for something major.