Military action
Too many mouths to feed, and not enough fertile land to farm under the mountain. Before war broke out, some willingly left their ancestral home and migrated North of the Ancestral river in the mainland but most refused to leave. It was then decided that each house would send half their decedents to the mainland to start anew. Out of the eight ancestral houses, the two largest; the house of Berg, and house Halny, refused to send half their descendants to the mainland. Thus war began.
The war lasted for ten years; within the first six the house of Berg was fully driven into the mainland but continued to fight nonetheless. Needless to say, the Berg's and all their decedent houses lost and founded the capital of Haufsburg on the North West coast. It did however extract a heavy toll on those that still lived in the mountain, and the house Hanly was able to unite the rest of the dwarfs in the mountain against the Berg's and bring an end to the war. The dwarfs that were removed to the mainland eventually adjusted to living topside and became more perspective of the world around them, choosing to live in the hills regardless of of the terrain North of the ancestral river being infested with monsters.