I've always just gotten it without really knowing why. Looked up wowhead and you just have to make sure adds are aggroed onto party members or NPCs at all times, otherwise they will set fire to the village.
I don't know what to say then. I've gotten it, and my guildmates have gotten it on multiple tries without really knowing how or why.
All I can offer you is what I do. We split into three sides, as usual. We set the traps just right where the NPCs are gathered, so that if that place is unguarded, then the NPCs can hold aggro and the traps will be continually set off. If it is one of those traps that you use instead of setting them in a specific spot, we use them libreally. Although we 'split' we do rotate around to clear stragglers, help with yeti etc.