If you have the app downloaded, Google displays the What's New first. If you don't, then it displays the Description first. This is so what's most important can be shown first. A new user cares more about the description, a user who already has the app cares about what's new.
I just verified this, your app shows the Description first for me, an app that I made and don't have also shows Description first. All apps I've downloaded show what's new first.
There doesn't seem to be anything that can be done to change this behavior.