You are on to something here. I found this article (can't read the entire one as I am not registered):
http://select.nytimes.com/gst/abstract.htm...DA80894DF404482
"PRO FOOTBALL; With Penalties Down By 20 Percent From '05, N.F.L. Reverses a Trend
January 25, 2007, Thursday
By CLIFTON BROWN; ARON PILHOFER CONTRIBUTED REPORTING. (NYT); Sports Desk
Late Edition - Final, Section D, Page 5, Column 1, 1110 words
DISPLAYING ABSTRACT - The number of penalty flags thrown in the N.F.L. declined 20 percent this season compared to a year ago, a drop that reversed a trend that had taken hold in recent seasons. According to an analysis by The New York Times, 2006 marked the first time since 2001 that ..."