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.