Though I don’t know exactly what causes it, I found that performing hip flexor stretches, and hip opening stetches really helped me to achieve depth without but winking. 

Are you missing dorsi flexion at the ankle?
 Liimited ankle dorsiflexion can cause a valgus knee.
Yes it could be a hip mobility issue.
Tight glutes can play a role.

Keeping my head and chest up and stretched along with being able to rotate my hips/pelvis down inbetween my legs has helped reduce my butt wink. I think I still have one, but it’s been getting better.

I’ll have to look into the dorsi flexion at the ankle that Kaitlin mentioned.

Episode 352: Dealing With Old Junky Tissue

Squat Quick Test: Is it your Ankles or Hips?
Episode 313: Improving Ankle Range, Super Friend Addition
Episode 285: Sliding Surfaces; Ankle Range of Motion Case Study Part 1
Episode 285: Sliding Surfaces; Ankle Range of Motion Case Study Part 2

Pro Episode # 36 – Voodoo Floss Series # 3: The High Hamstring
High Hamstring Gnar, Trigger Points, and Up Stream/Down Stream
Episode 272: TJ Murphy Edition and Hamstring Stiffness

Apparently I start butt winking as soon as my hips drop below parallel…  🙁 If I’ve read this thread correctly this is generally an issue from tight hips? That is a known issue I have.

Are you seeing improvements with what you are working on with your hips?

I can’t tell when I do it Kaitlin, I was getting some tips the other day from a kettlebell master instructor and he drew my attention to it fast – I am supposed to not drop my hips below parallel until this is sorted. The thing is, “I” can’t tell when I am doing it  :”>

I only butt wink at challenging weights.  So I think its a weakness/strength issue

Catherine–
If you aren’t able to recognize when you do it then squat to a depth that you know it does not occur.
Continue working with your spine&hip mobility, and check in with the instructor you mention above every week or so for a check in on your technique.
Spine first as the spine is the carriage or chasse for the primary engines of the shoulder and hip.
Disorganized at the pelvis hip function is decreased.
Changing the position is the way to improve the function.

Daniel–
This is a technique error that shows at the upper limits of your capacity.
Your body is able to buffer the deviations in technique to a point then it is not able to any longer.
At lighter weights your body makes compensations that you may or may not be aware of.
Carl Paoli, GWOD, has a great video on blocking movements for quality squats.
Blocking Movement for Quality Squats

This is a great video to start correcting your squat technique from the beginning
Work within the limits of your ability. This will improve as you increase your mobility and address deviations in your technique.