The original poster says that he died to "Arcing Slash." I am assuming he looked at logs to acquire that information. If the logs truly do say he died to "Arcing Slash", then I'm inclined to believe them. You are certainly right that it could just be visual lag, but I don't think the poster would have mentioned a specific attack unless he checked logs.
The reason I believe this to be the case is because last time we did kephess HM, when we wiped I was one of the last alive and I remember getting his debuff, standing in a circle with him not aggro on me, and dying in the purple circle yet my death message said arcing slash.