Uh, did we know that Black Death did or did not enter Japan at that time ?
1) they relatively close to China, and have many sailing vessels go to each other, and China did suffer from Black Death 2) 1350-1650 is Warring States period, in make sense that chaos generate by so many death helped cause destabilization and state collapse 3) It also time for for Ikko-ikki, which means peasant and religious radicalization spreading, another characteristic of society brought to chaos