Much in the same way that the GW2 event system works. Events are linked to an overall theme of the racial lands or the zone you're in and while after a time they may repeat it's cleverly designed to seem like a tug-of-war between the NPC's.

It's an overall well thought out system that's more organic feeling to the world than areas popping up just because that's where they spawn.

I can't explain in under 1000 characters what I mean unless you've played GW2