These are dynamically generated by the game's systems, and allow you to hop into fights alongside swarms of other players.
Guild Wars 2 isn't a traditional MMO, in the sense that it replaces a lot of boringly structured fetch quests with more emphasis on live events you can stumble across while exploring the world. These are dynamically generated by the game's systems, and allow you to hop into fights alongside swarms of other players. The fighting is more dynamic than most MMOs too, as you dodge out the way of enemy attacks and aim your own, as opposed to being locked into an animation.