If a poor initiation or lack of mobility isn’t your issue then motor control (the ability to control your body) is. The fix for this is simple: do squat therapy on a wall everyday for 2 minutes until you can get your butt on your heels without winking. It’s incredibly simple, but amazingly effective.
Squatting is one movement every human needs to do. The loss of the ability to sit down and stand up means a loss of independence as well as a saggy bum. So tomorrow have a friend or a coach check to see if you butt wink and if so use this article as a guideline to fix it.