I think, and that is a opinion...

You need to kill 2 trolls and the Elder to cause the event.
This is usually a typical activation as its rare to get to the elder without killing a few trolls , especially if you like the gear.. I love the hood and cape at least.

I suppose its possible to avoid all trolls and kill the elder, and thus logically avoid the troll event. but then you would get more swamp events.
I would GUESS its a kill elder then get trolls event, as its expected you would have nabbed a few trolls before.