My dogwood is dying from the
top down?

Dogwoods do not like full sun, they live in the forest understory.  It looks like the tree in question has gotten too much sun. If possible move the tree to a place where there is more shade especially afternoon shade.

Related Videos:
How do I get my dogwood tree to bloom?
Why does my new dogwood have brown leaves and look like this?

Related Resources:
Dogwood
Dogwoods for American Gardens
Growing Dogwood