In the 1400s, prior to the tokugawa unification, why did japan experience destabilization?

Before the unification of Japan, Japan was a feudalist country. They are divided into different daimyos despite having a shogun to govern them all. The country was constantly at war because of the land territories. Japan was only united when the Tokugawa Shogunate has finally resigned and surrendered his power to the new emperor, thus the start of the Meiji Restoration