Looks pretty good from what we can see in the vid. Good depth, looks like your knees are ok, stance is ok, back is arched during the rep. Only thing is, as jigga mentioned, you do some sort of weird thing where you don't completely extend your hips and stand up. It might be that you're concentrating too hard on arching your back to the point of hyperextending your spine while your at the top position. As you bend down, your spine flattens out into a more natural position. It looks like at the top you might just be hyperextended and your glutes arent fully flexed. If that makes sense.