Once i almost died in Brawlers guild and another player joined in with his boss, and his boss was like one of the first of brawlers, and that one died and i got ported out and it got counted as a kill, sweet for me x)
That happened a lot when the Brawler's Arena first started, probably for the first week or so. A guildie got credit for a kill they shouldn't have, other's didn't get credit. I'm sure they will fix it soon enough.
Seemingly there is a bug where if you use a specific item whilst fighting, it allows the next in queue to start their fight, and the previous person gets kill credit if the new person kills their boss.
Folks were discussing it in /s on our realm today.. needless to say best bet is not to try it!