Why are there two associated Dhamma/Dharma Wheel forums ...
I think you are seeing religious evolution in action. Various boards have differing characteristics that affect their ability to survive. The Pan-Buddhist boards keep popping up but they tend not to function well, because it just takes a handful of dogmatic sectarians to tear one apart, or start so many fights that the moderators burn out.
All it takes to start a sect fight is for one user to have a really bad day, sign on in an angry mood, and post a single nasty rant. This begins an avalanche effect in which those who feel mostly strongly on the topic join the fight first, then those who feel not so strongly are provoked by the inflammatory rhetoric and THEY join the fight, which then exceeds the patience of more users, which leads to more rhetoric and so on until either the whole board lights up in flames or the moderators kill the discussion.
So there are really three factors that affect the chances of flame wars.
1. Loose moderation
2. Sheer size of the board
3. The presence of many factions with many different views.
So a small board, confined to a single sect, and tightly moderated is long lived and stable.
A large loosely moderated board accomodating many factions is extremely unstable.