Paladin and/or Alignment threads are often very volatile. The current one is unusually long lasting, though it started to smoulder a few times. I think it is because it involves discussions of right and wrong, discussions which easily veer into real world politics and/or religion.
Any rules question that doesn't find a satisfactory answer in the first 20 posts or so often either starts going in circles or going off on a tangent. The circle can easily become a death spiral, usually at about the time when one side gets frustrated and accuses the other side of introducing house rules instead of following the RAW. A long rules thread with a small number of posters and a small number of views (less than 10 per post) warrants very careful watching.
Edition war threads are also flammable, but I haven't seen much of that lately.
One thread topic that seems almost guaranteed to get closed is when a WotC official speaks. That really brings out people's distrust of authority, and accusations of bias and decit are soon flying.