[Brut]

Chck logs if you got hit by a lifetap dot before that. There was a long standing pvp bug (which I think is still the case) wherein Vampiric Curse line doesn't make an effect on your list, but still exists there. Basically your first slot does have a debuff, but it just doesn't show up. So when the next dot hits you, it goes to the first slot, but it's actually now on the next open slot (the last one probably in your cse).