It's just hip flexibility, the classic style it to be more upright but if it works for you who cares. More upright means that you can drive you body forward into the kick more. Leaning back means that you pivot more around your standing leg, with your upper body being the counter balance. I have more flexibility in my left hip than right, so i stay upright on my left kick and lean back on my right.
One thing one might look at tweaking is your upperbody position. Look how twisted your spine is. Your shoulders are trying to go left and your hips are going right. Instead of trying to swing your arm back, think about if you were kicking underwater and were swimming forwards with your arm. If you look at Jordan Watson here, he has his shoulders and hips all in line and and traveling in the same direction.
Everybody has there own thing, and it might not be for you but i thought id point it out.
Edit: another one with Yod but his arm position is a little exaggerated and in front of his leg