I found that 2 shaman DoT spells have a repeating spellcasting animation every time the damage ticks. These spells are Flesh Rot 1 and Hoarfrost 1. If both DoTs are active then the Hoarfrost 1 animation usually takes over, but if only Flesh Rot 1 is active then I see only that spellcasting animation repeating. It appears only to be cosmetic.