Some addition to Jdale's very good points:
- are they really refugees? Maybe they are spies sent to divide the clans, maybe they aren't but someone starts to think they are.
- maybe there's something they need to regain control of their own reign. It may be an actual object like an artifact, relic, or symbol of power long lost, but it could be something less tangible, like help to infiltrate their reign and raise the people, orgnize support to rebel groups in the reign or even start a military campaign with the help of the clans.
- things starts to go wrong, like plague or famine, and people starts to blame the refugees (depending on your religious system this could get fanatical nuances, just think about how black plague was handled in Europe in middle ages). They may have brought it, for example some of them was already ill, or maybe someone has a cursed item that's bringing doom upon them. Maybe they are innocent, and someone pulling the strings is using them as a scapegoat.
- Culture and religion, as pointed out by Jdale, are a gold mine here. Maybe the refugees are highly civilized and always treated the clans as barely human. Maybe their religion impose them to treat tribal beliefs as demon worshiping, or at the opposite the clans treats their religion as silly and alien and offensive to their gods. Maybe refugees are citizen or highborn, inept in everything that is needed in a more barbaric environment and they struggle to find a way to be useful to the clans. Keep in mind that these differences can work even if the refugees are less civilized than the clans with few adaptations.
- the refugees brings something new too the clan that starts a change in the power balance among the clans. Depending on the availability of resources in your campaign it can be very different things. If your barbarians are bronze age, maybe they bring the secret of working iron. If they already have iron, maybe they bring steel and so on. Maybe they have better techniques for agriculture, better engineers for tools or architects for buildings and fortifications. If your barbarians dont't use magic, maybe that's what they bring, or better and different ways to practice it. Maybe barbarians didnt have mentalist, or hybrid spell users. One or more of these changes could make one clan more powerful, and this could bring tensions or even open war.