When most blogs publish they "ping" the blog directories to update and list their posts. That’s partly why the blogosphere is so speedy. The traditional search engines take a while to get round to listing new pages.
Here’s "pingomatic’s" chart of the daily pings it receives. You’ll note a steep climb in September. My best guess is that it’s the Katrina effect which really got people blogging like mad. Big news, especially in the USA, is productive for blogs.