It used to be possible to "KS" an elite in the sense that if someone swooped in on the map after the elite spawned and outdamaged you, they would get the extra bonus box (for a total of two) while you would get only one of them. At this point, as long as you were the one who "summoned" the elite boss (ie killed the mob that triggered the event), then you get two boxes as well. In other words, it used to be a thing, but now it isn't really.
Other people might consider it KSing if you kill an elite they are trying to "hold" for their friends/guild/alliance. It can be especially annoying in the arcane river, since elite boss HP doesn't seem scale up the same way as mob hp does there, so most people training there are already strong enough to finish off the elites in a matter of seconds.