It could be someone just thrilled to shreds to be able to be a "source" of a gossip outlet.
Or, it could be someone who might not even realized she was being pumped for info by a reporter. "Gee, it's so nice that you took me out for coffee, and sounded interested in *me*. I've been so sad since hearing the news about my friend... Oops! You're a reporter, aren't you? Please, don't tell anyone I told you anything about Elizabeth." Hence, "spoke on condition of anonymity".