When you begin your descent you are focused on not moving your knees at all. Then when you get close to the bottom you can't hit depth without sliding them forward. Bend your knees early. You are doing this on some reps (sets 70 thru 90 looked better). I don't think lowering the weight will help - your worst sets were at 60kg. You probably need someone watching every rep and coaching you in real time. When do you see the SSC?
Here's a video to explain what I am talking about
:
Setting the Knees Early During the Squat | Steve Ross