@zinhead you are correct sir. Emory was also helped by a huge gift of Coca-Cola stock, but the real difference has been the rise of the South in population and economic power. As that has happened, the better schools in the South naturally reap the benefits of better applicants, more donations, more research money, etc. Emory is the top dog in its area, and naturally moves up.
Other factors can come into play in addition to regional economic activity. 30 years ago, Duke was a very good school but primarily regional in its appeal. The high profile success of its basketball team arguably helped just as much as the growing North Carolina economy to propel Duke into the top of the desirability rankings among high schoolers. Same thing with Georgetown. Georgetown has ridden the economic and political tide as the best college in the Washington DC area, as that area become more and more rich and important. But the high-profile basketball team played a huge part as well.
Who knows what the next thing will be? I suspect that some college in the growing Mountain States is going is going to move up over time, perhaps one in Denver or Phoenix. I also suspect that Texas schools will move up.