I'm guessing back rounding at the bottom due to lack of mobility and going too deep is the cause as well. Not very many people are flexible enough to do an A2G squat without rounding their back when they first start out doing A2G squats... myself included.

Take a video of your squat. I'm almost positive that its lower back rounding. If you squat without weight watch your side profile and if you see that your butt "winks" at the bottom of your squat or starts getting pulled under you... Then you know you have a mobility issue. And I'm going to guess thats its due to tight hamstrings. But it could be a combo of other things as well. (Ankles, hip mobility, etc).

BTW, good post Eric. Way to rip on Rippe.

IW