One of the campaign winning conditions of Chaos Invasion is for dwarves to be wiped out.
In the lore, the dwarven underway was used to invade & wipe out the lizardman factions who were a primary opponent to the Chaos, and it's (afaia?) implied that the Skaven (and orcs?) were manipulated into doing their bidding
Soo, make every default-dwarf settlement, when occupied by a non-dwarf faction, have a monument they can build at tier 3, whether it's a minor or major settlement, that gives a % boost to underway range when starting or ending in a province with said monument, that stacks globally.
So if Skaven or Orcs are cowed into an alliance with Chaos & enter into a war with the Dwarves and Lizardmen as a condition for being spared, their implied reason for their demanded hostility with the dwarves is to cross their underway-capable vassals to underway from the old world to the new world.