People may use, say, religious or ethnic labels, but often the underlying cause is unemployment. Economic growth is often the glue that holds a society together. If everyone is doing well, so there is little need for civil unrest because the citizens are too busy enjoying the fruits of their labour. There is more to employment than just acquiring an income. Employment is an entree into society.
Social exclusion leads to despair and alienation.
If people are excluded from society, then it is no wonder that they turn to violence. There is no civil war in any country which has full employment. If work for all who need it is a basic feature of maintaining stability in a society at peace, then it must also be an objective for reconstructing a society after a conflict. After all, ex-fighters need to be given an incentive to help rebuild society. Peace brings the risk of losing that status - and the prospect of unemployment.