old post i know but... its random, totally random. If it's in window, it will just spawn every so often and then despawn after 30 mins if not killed.
I know its random b/c had it spawn with a toon standing at zi and nobody else in zone several times. Tried all the trigger traps stuff many times to no avail. Although I can see why people would associate it spawning from a trap ( there are so many you are always stepping on them ) No real rhyme or reason to it, just need to be there when it spawns.