I think it's just a graphical effect built into the Viking Archer model. I did a search on the NIF files [unit skeletons] and the Viking Archer has a node [important subsection] for a "SuperSpray flame ity-Emitter". I have no idea why it is named that, but if it's part of the NIF then it is graphical only.
England uses the Viking model because there is a limit on the number of art definitions a mod can support (this is inherent to Civ4, although I don't know what is the limit and I suspect it is more the total complexity of all included art definitions; what I do know is if you go too high, you get crashes via Memory Allocation Faults). To maximize the ethnic diversity, I try to have two or three closely-related civilizations use the same model.