No, no, we wish. Actually, it's the monsters who do that.
Call to arms is a feature implemented to make fights harder. What happens, is that a creature that spots you sends out a "call" that draws all hostile NPCs in a certain radius. It was put in to prevent players from drawing one monster at a time out of a group and picking an entire horde off singly.
I like it, actually. Instead of having to go into every corner to clear out monsters, they come right after you... and into the Web.
Some monsters don't appear to use the call to arms. The undead in the Vale are an example of that.