ADDED: I guess this is the whole answer I need (from the linked page):
If you make a large number of posts in a single day, you will be required to complete a word verification for each one, independent of whether your blog has been cleared as a potential spam or not. If this happens to you, simply complete the word verification for each post, or wait 24 hours, at which point it will be removed automatically.
This restriction is in place as much to control the load on our servers as to prevent explicit spam. Therefore, there is not a whitelisting review process to exempt individual blogs.
But why was today the first time I triggered this device? Was yesterday my biggest posting day ever?