AvatarNathan Richer

yes you do buttwink right as you cross horizontal with your upper legs.

lots of reasons and solutions to buttwink. 
1. it is hard to tell how far apart your feet are, but it will be easier if they were just a tad wider than shoulder width. once you go wider, it will get harder. it will also get harder if the feet come closer together.
2. do lots of hip openers with band, and work a supernova/lacrosseball into the glutes, quads, hams, adductors.
3. your feet look like they are pointed out. it is ok to point out a little bit but it looks like your arches are collapsing which makes me think that you are not driving your knees out strongly enough. driving your knees out will create space between the legs for your hips to drop between. when you drive knees out, only go as far as not letting your big toe off the ground. so while knees out is a good cue, you really want to screw your feet into the ground to create torque, which also drives the knees outward in reaction.
4. lower leg needs to be worked – calves, heel cord, posterior tibialis, anterior tibialis, peroneals. 
5. hows your ankle dorsiflexion. it looks ok here, but you will want to test with feet straight instead of pointed out. the knee should track slightly to the outside towards the 4th little toe as you do ankle movements.
6. to do the squat properly really requires a mobile t-spine. i’d get on a double lacrosse ball or gemini and make sure you have mobility in the t-spine, which may also encourage a buttwink if too stiff.
7. one last thing – set the femur heads to the back of the socket! get on the ground with band distracting to the outside or rear. then with one knee down, try to push the femur head out your butt behind you.  do this for 2 min at least each side. if the femur heads are biased to the front, they will run up against the front of your pelvis and cause the buttwink.