There are other MMOs like Rift where there are factions of NPCs that will fight each other on site. There isn't really anything like that here.

Probably two exceptions like already mentioned though:
1)Fates: These are scripted events. So what happens in the fate is just a little mini story so the NPCs can be hostile to each other sometimes if the mini story of the "fate" demands it.
2)Leves: These are scripted too. Same deal

But in general? There aren't any hostile critters that don't like each other... Even the A and S Rank hunt mob NPCs are neutral to everything but players.

That and, well, NPC mobs don't really "travel" anyway. They pace in a little short "train track" of walking but, they aren't normally even going to see anything beyond their buddies...