Hi K
That is good noticing. 😉 You know, this is a natural response to trying to get deeper into the pose. When you have your feet in a bit and knees positioned outward – then the hips tend to have an easier or less resistant time opening up in the backbend.
If you want to enhance the ability to open your hips then try to resist this movement. And keep the widening of the knees til second set which is often when it is invited by your teacher.
Simple answer. You are not strictly doing anything wrong. You could certainly improve the flexibility in your hips by keeping everything firm for the first set.
Namaste
Gabrielle 🙂
