Some are group kill chains other are only for you. So sometimes you will only receive the exp buff instead of the whole group. I'm really not sure on what set's these chains off as they can happen when I am luring up in PT and haven't killed or attacked anything yet.
EDIT : Don't forget those people who exploited got bugger all in the way of punishment so if you find a bug pm me, I'll exploit it. (And get others to join in as well) I mean cmon guys, you need to punish cheats, exploiters, bug abusers etc otherwise everyone will be at it and server will die... Look at the Firm for example, they are still killing a very bugged and broken boss... Yims, or is that sorted yet and the clone actually behaves as it was intended? They also abuse a 12 year old warring system. But this is fiiine cos it's TF.
Wonder what useless ballocks tomorrows update will bring, a new cave? Some new monsters? How bout some new skills? Or wait, how bout fixing and sorting **** already in game? No? OK, I'll **** off then.