Military action
After being cast out to live independently as a race, inequalities started to surface between the two dwarven kinds.
As the seperate races of dwarves came to share a world with one another, the southern dwarves discovered an injustice done to their race. The high dwarves discovered an "uncivilised, unintelligent, belly-high" version of themselves. This exploded into a conflict with an immense arena.