As far as I know, the understanding from the Sirius side of the universe on the Humans arrival is that the Dwarves constructed a doomsday machine to fight back in a war they were losing against the orcs. Activating it resulted in a huge catastrophe that decimated many races and star systems in Sirius, especially the dwarves and orcs, and also somehow brought humanity and something called "radiance" to Sirius. Radiance being a sort of "voidstuff"radioactivity that corrupts and twists man and machine. The Radiance Machine pulls from a lot of Dwarven archtypes; Heart of Lorkan or the Balrog in Moria.
As to how the radiance machine led to the nexus of humanity's civil war being yanked wholecloth into Sirius, that's a mystery of the setting.