He sets out his theory here.  Briefly:  highly ranked schools have enrolled fewer students during the decline, rather than taking the more students with weaker credentials, in order to maintain their rank in U.S. News; the result was more students with good (but not top-flight) credentials available for lower-ranked schools.  To which I say:  maybe.  The decline in enrollments at top schools has been small, and many have seen declines in their student credentials anyway.  But it's an intriguing possibility!