When you're performing any kind of squat exercise, besides doing it with proper form, balance and appropriate weight, you also want to AVOID THE CLASSIC "BUTT-WINK."
Like, you didn't already have enough problems already. Now you have to worry about being called out across the gym for doing a butt-wink.
Notice how the gluteus maximus (or butt), is pointed out and the lower back is concave? That's what YOU WANT! What you're trying to avoid is rounding the lower back and pushing the butt downward and in.
Think of it like gears. Your back is a large gear and your butt is a smaller one. Both should be moving together in a way that tucks the lower back in while pushing the upper back out and the butt out.
Does that make sense? Not really? Watch the video below to get a better understanding of what I am talking about and how STRETCHING IS YOUR ANSWER!