Is there any reason the Orcs are assisting the Nords? I can understand the Nords trying to expand their territory into Morrowind, but have the Nords and Orcs had any kind of military alliance in the past? I'm not much of a lore buff
Both cultures love to fight. I guess some young advlt nords, during their initiation ritual into advlthood by going into their first raid against Morrowind, brought some friends from the nearby neighborhood of Orsinium. And what better way of making friends by inviting them to their ritual right of passage into advlthood