I agree. Warsong had bigger population before Warmane, but "who pushed whom" is just your assumption. I don't think ppl would just leave cause there's new kid in town.
It's really a mystery, no one can tell what actually happened cause there's not enough data to make a conclusion. You think you have it? :-) Let's hear it. Just remember, "I heard" and "I saw" aint gonna cut it. It's just your experience and not enough to make statistics about hundreds of people. I think we can only speculate what happened. Maybe both Molten and AT players left for God knows what reason. Maybe Molten lad and AT maiden just left WoW for few days. They met, got married and realized all they need is each other :P ....Hehe, maybe they have kids now, to busy to play WoW.
My guess is that Blackrock was the reason. During Warsong golden days, there was no WotLK pure PvP server. Now, there is. Maybe Cata PvPers tried it and some of them moved to WotLK.