Quite simply they're playing to the crowd. That's what gets a reaction, so they do it more. Skinner box conditioning at its simplest.

Same outside the hardest moderated groups on Reddit. Threads that form a distinct pattern every time. For internet points, or twitter followers, or chasing meaningless numbers. :)

Sarcasm and smart works best if used judiciously.

How do you design a system to encourage quality debate? Going on the few places that have had quality debate (Here on HN mostly, Usenet prior to the inrush, well managed forums, Livejournal if you were careful), I'd suggest assembling an audience interested in discussion. Forums used to manage it.

Heavy moderation, not limiting post length and minimising buttons for gathering the lynch mob (I mean buttons for engagement) seems like it helps too. :)