That butt wink should be of concern because it indicates full flexion of the lumbar spine. Winking leads to disc problems. It is a dysfunction in the ability of the back muscles to activate at that range of motion which causes the winking. It is movement dysfunction and should be avoided.
So, how do we deal with it, that is the question. First of all only squat in the range that you don't wink. That way you will be training in a range of safety, proper function and grooving movement that is correct. Eventually, your range of motion will increase as your ability to do so correctly increases. Hamstring stretching may help, but you have to be careful with this because often hamstring tightness is a compensation for pelvic instability and stretching can upset the apple cart. I don't take this approach but focus on lumbar muscle activation, strengthening and improving endurance.
1. Face down, Hip free position
2. Back board straight, that is place a ruler and contacts should be tail bone, between shoulders and back of your head. This posture will be kept - at all times- throughout the movements.
3. Now you are ready to begin. Drop down to a bent, at your hips (still with a board straight back) position, activate your low back muscles a little, and come back up to the point where your are straight, like standing. Hold it, 5 - 10 seconds, breath during the hold.
4. Return to the down position and come right back up. Absolutely no hanging around down there. This should provide a very short relaxation time for the muscles.
5. Repeat to fatigue.
6. Fatigue (this is important!) is low back pain, can't hold posture, pain somewhere else, excessive shaking, and the last is can't do it.
7. Don't increase the hold times, increase the reps. If you want to put on a bigger hurt, perform two forced exhales that last about 3-4 seconds each.
8. Finally, recognize signs of fatigue and respect them, highly.