Purple Robe Locust tree dropping leaves from the top...What's up?

I have a three year old Purple Robe Locust tree on the East side of my house. It's been dropping leaves from the top for the past three weeks and is now bare except for the bottom two branches. It appeared healthy before this. I deep root fertilized with Superthrive when I first noticed the leaves dropping. The bare limbs appear alive although they are leafless. I'm concerned about sunburn on the bare branches as well as the general health of the tree. Any ideas what's causing this? I live in Arizona.