getRootElement() or root will trigger that event, and therefore call the function when any player changes his nick, as all players are childs of the root element. (Check the Element Tree in wiki)
If you set the localPlayer as source, it will only trigger if the localPlayer changes his nick.
You only have to be careful and checking the wiki what the source of an event will be. (players, vehicles, markers..)
Usually its best not to set root, especially when working with ResourceStart events.
Bonsai