Back in the day, rush at my school took place in the fall, before classes started. I noticed that odd years tended to have good numbers, and even years tended to have bad numbers. NPC quota was in the low 30s in odd years and the mid 20s in even years. The pattern seemed to hold true as well for fraternities and independent living groups (ILGs - sort of coed fraternities).
Starting in the 2002-2003 school year, rush was deferred. Fraternity and ILG rush was held in September, and sorority rush was deferred to February. Fraternity and ILG rush was therefore happening a few weeks into the semester, when classwork starts to get intense and exams start being given, so a lot of guys were too busy studying to come out for rush. As for the sororities, a lot of women are involved in a bunch of other activities by the time spring semester rolls around, and they don't know if they can make time for a sorority, plus it's their first semester on grades (fall semester freshman year is pass/no record) and they worry that they can't keep their grades up AND pledge AND do everything else they're involved in. I think quota was only 21 last year.
Fraternity numbers were down again this year. I'll be very interested to see what sorority numbers are like.