They claim this is because of their focus on Kubernetes, but I have to wonder how much of this has to do with Microsoft acquiring Github. That aside, that's a tough slight for AKS.
To be fair, AKS wasn't generally available until about a week ago. And while in preview, there were some significant issues with AKS (they themselves recommended not to do production deployments on it for the preview period).
I don't think moving to GKE implies that AKS is inadequate. They could just be choosing the best of two excellent options.