I'm just relaying anecdotal evidence, and it's only for one field, but I'm sure it's true for CS because I just spent a year looking at various universities with graduate programs. Pretty much all of them are the same way.
Also - the recent downturn in the economy, particularly in the tech sector, is good for creating American Ph.Ds. When the economy's really hot and you can get a good job right out of college, why go to grad school? But when it's not great, you might as well get your MS or Ph.D. So there were probably more applicants two years ago than there will be next year, which means more advanced degrees are in the pipeline right now.