I also saw Charly's old videos. Amazing to see the progress form then to now, not only in weights on the bar but also how much he's grown.
I kind of have that over-extension problem too. I think its partly a technique problem, you simply squat inefficiently and have learnt a faulty movement pattern, and partly its due to weak muscles, most likely the the glutes, quads and prob a bit back. My trainer has said weak glutes are one of my weaknesses so I'll do more work for it in the next training cycle.
Yup, you def do from seeing your videos. I agree, It's so hard to break a movement after doing it for a year or more because all those muscles that support the movement are strong. Here's just a little information that might help -
When I had Greg Nuckols he told me I need to strengthen my quads and glutes. I said OK and did a ton of stepups, highbar squats, paused front squats etc. My quads and glutes did get stronger and it took me to 300lb squat but at that point I was back at my original problem. From there I could strengthen my glutes and quads more but I stepped back and thought why is this happening. Didn't really figure it out until I went to that PRI class, what's funny is that it was a room of 6 old ladies and It applied directly to powerlifitng. Thought I was crazy for a second because everyone I trained with squats like that. But Chris Duffin explains it better and it's easier to listen to someone with a record squat than someone like me who is DYEL and squats high 300s.