The category is added dynamically by a tag extension. Any page that uses the tag <foobarblat> is automatically placed into category "Foo". Doing it at article-save time would not work, since the tag could be present by transclusion (e.g., added later to a template used by the article).
We use this technique to keep track of all articles that use some resource-intensive tags. (For example, accessing external databases.)
DanB
-----Original Message----- Thomas Dalton asks: When are you adding this category? Can you not just concatenate the wikitext onto the end of the article when it's saved?