The Eighty-Twenty Solution

I've noticed that some weblogs have turned into ad-hoc discussion fora. John Wiseman's Tranquility Bay has become an ad-hoc forum for discussing that facility, and then there's the absurd comment threads. I thought John's Like Beyoncé and Jay-Z took the cake, but Simon Willison seems to have him beat. What's going on here? You'd think that most of these people are arriving via some search engine, probably Google, but often, the page is pretty far down the rankings, but sometimes the post referenced is relevant only with respect to the (probably ironic) title. There's some odd social quirk here - people show up and decide to start talking about something almost totally unrelated, and the change of subject takes hold. I think this an instance of "autistic" user behavior; where someone socially inept makes an inappropriate subject change, except that the normal social controls don't hold, so the normal social response doesn't have any effect. It only takes one off topic comment to totally change the subject.