The early end to the regular Fall book everywhere probably had a lot to do with it.
In big PPM markets, the regular fall book showed a moderate effect of the all-Santa format during the last two weeks of sampling, which included post-Thanksgiving holiday music on some stations. The numbers you're hearing about now, with drastic jumps in AQH 12+ for the dominant Christmas music stations in places like NYC and LA, were from a special Arbitron book covering the period from December 9, 2009 to January 6, 2010, which was compiled and reported only in PPM markets. The rest of the country, including Buffalo and Rochester, wasn't gathering ratings numbers at all during those four weeks. This special report was probably designed to give advertisers and broadcasters a real gauge of the impact of holiday music on those stations who program it. Hopefully for everyone's sanity, the strong results of a few stations won't cause everybody to jump on the holiday music bandwahon next year. But we should probably be a little afraid....
One more factor; in markets like NYC, one station became so totally identified with Christmas music that it got substantially all the action from people looking for that kind of music. Buffalo split the holiday music audience between a couple of stations, which then proceeded to cancel each other out and move the ratings needle back toward normal.