Goodness, that is not what I had expected!
I was thinking of something more gradual. I am no expert when it comes to evergreens but I know a bit.
My first thought was a root problem that was causing layer by layer to die back, but then why is the top part so nice? It should not be an insect issue because with an insect problem, no area should be unscathed.
Not a fungal problem for the same reason.
Honestly I think it's beyond hope.
And I am clueless.