This is one of my go-to movements for teaching good upper body posture and is a great  exercise to improve cuff strength. 

It however is often done incorrectly and for high volumes at that! 

The two biggest mistakes are:

Viewed from the front the person looks okay, but stand to the side and you see them in a ton of extension. 

Viewed from behind at the shoulder blades, you shouldn't see excessive scapular retraction. The movement doesn't have the person moving much past neutral and should mostly be the ball moving on the socket in the shoulder. 

Here you want to get them into a neutral spine throughout the movement.

An easy way to fix this is to perform them supine over a foam roller or half foam roller if available. The roller helps keep your back neutral and as an added bonus gives you feedback about the second issue of being overly aggressive with retracting the scapula.

When you see the shoulder blades pinching aggressively together it doesn't sync the shoulder blade and the humerus which can lead to dysfunctional movement patterns and muscle recruitment. 

You ideally want to separate the motion at the humerus before the scapula moves at the end. If you don't feel it burning (mostly) behind the shoulders, you're not doing it right.