Common Squat Mistakes Series-Butt Wink
Apologies for the head being cut off! And for the ambient noise in the background...I'll try to improve the quality of these.
I think some additional text here is needed. The butt wink can be caused by many things, but in my experience, it's mostly a result of lack of range options in the hips, forcing the pelvis to change angles. The rear of the pelvis drops underneath the body, forced to do so because after a certain depth of squat, going any lower is only possible this way, if hip space isn't created. After all, what's directly underneath the butt (pelvis, duh.). Creating hip space by moving the femurs laterally allows the pelvis to maintain a roughly neutral position as an athlete descends in the squat. It can also be related muscles attached to the pelvis, as this video talks a little about, but fails to mention thoroughly that hamstrings and even tight lats can contribute.
A case by case basis is needed to truly find out what the cause of a person's own butt wink is the result of, but I feel fairly confident that a regular regimen of these simple mobility moves will improve, if not eliminate, the wink.