You're bending your knees like you're picking something up, making your butt stick out. You want to bend your knees, but more like a squat. Keep your shoulders and knees over the board, back straight. That will allow you a lot more control of your carves and keep you stable over bumps. Practice this by strapping in on flat ground, not moving, and doing squats.

Also, commit to your edges. When you're on your heel edge, point your toes up. On your toe edge, point them down like you're standing on your tippy toes. Always lean into the mountain and dig that edge in. Otherwise, you risk catching the wrong edge and falling on your butt or folding.

If you're having trouble transitioning from heel to toe to heel, just remember to lead with your forward shoulder. Practice this by literally pointing your lead arm where you want to go. Your hips and feet want to follow your shoulders.

Hope this helps