Some of the kiji, having not been summoned solely by Gaia, started developing empathy and a deeper understanding of the world around them they fought to save. For some, this sympathy for the humans they slew meant their own deaths, as when they returned to the Core without any collected energy, Gaia locked them out and they eventually faded away.
The energy of these faded kiji condensed into lifeless balls, which were later collected by other kiji and returned to the Core, to be summoned into new kiji later, after the War. These are the Post-Thinners.