In contrast to the seemingly endless debates about other powerful empires, the Fall of Kubii has a single clear cause: structural collapse.
Cause
Like most Haaru societies, Kubii was organized in a branching hierarchical fashion, generally organized parallel to the anatomy of the tree. However, this model of government can be quite inefficient on a large scale, and Kubii was a kingdom of tens of millions. After years of dominance in its region, the strength of that central power was increasingly flouted by the considerable economic and social power of its Cladarchs, some of whose branches had populations rivaling other trees.
By the reign of the ineffectual GeKubii Kai Matoanu Samatsak the king was largely a figurehead mediating internal disputes between Branchlords. In many cases, he lacked the power to unilaterally enforce his rule over them, requiring the cooperation of similarly disagreeable Branchlords who usually demanded much in turn. This system of favor-trading was a difficult way to reign, leading to compromise instead of strength, independence instead of unity, and chaos instead of coordination.
One major point of contention was Graft Administration, the management of attaching symbionts and grafts to the tree and its branches. Each new graft puts an additional strain on the structural integrity of its branch, as well as on the overall nutritive balance of the tree. Overgrafting can cause the sapwood to become desiccated and weak. And as the Trunk Excavations become more complex, the precarity of their structural support must be more and more tightly managed.
Collapse
The Fall began with a partial collapse in Simaya, one of the largest and lowest branches in the tree. Simaya itself was overburdened with development, having major trade and water-power infrastructure where its twigs reached low over the Suaa. trunk. Once Simaya fell, a chain reaction was begun that brought the whole tree down within hours.