Although the green wyverns have been included in the orcs forces for a short time before, we needed to take them down as their animation of death wasn't improving the immersion.
Since then (a few months ago) we've been continuously testing and improving them.
In the current state, they are using adjusted ragdoll physics that make each of their death unique. We tried to achieve the most realistic and immersive movements.
That is why the wyvern falls down using the initial velocity of its fly, the force of an arrow and gravity combined to achieve that effect. While this creature is falling down you can notice their panic wings movements that do not prevent from the unavoidable fall.
Here is the gif: