After a thousand years of peace, the magical barriers which have separated the world of humans (Elder Soteria) and the world of the fae are breaking down. The fae are eager to return to their lands. Since the barriers are still somewhat up, the elves and dwarves cannot return all at once, but rather the most persistent and potentially most upset of each are the first to return.
The dwarves and elves were not meant to be banished, they helped put up the walls which banished them unknowingly. They are angry at humans and feel betrayed. They would like their ancestral lands back.