When you disenchant an item, a loot window appears and you can loot the essences/dusts you disenchanted. If you have selected a NPC at that time, Thottbot probably thinks you killed and looted that NPC (because there's a loot window) - this is why so many NPCs seem to drop magical stuff, although this is not true, of course. Same goes for the omnipresent Clam Meat and Morrowgrain.