Question about Politics (Fujii/Mori-- late 1500s)

Can we shed some light on the politics of Hiroshima/Okayama in the late
1500s.... why did Kogen throw all his resources into dying with the
Amago why the rest of the Fujii family were vassals of the Mori ?

I understand the principle of a family sending its members to
different Daimyo and such... and of the honor and loyalty to one's
master... but I am also wondering of any other less obvious and
possibly less "fantasy-sounding" reasons why the Fujii home base near
Yoshii would fall to the Mori Army while.... within the Mori Army
itself, you had Fujii commanders and Fujii shinobi/samurai.

Any insight would be awesome.

Thank you.