Everywhere I read it always says its weak adductors that cause knees to cave in when squatting. Why wouldnt it be weak abductors since abductors are the muscles responsible for moving the legs out(away from the midline of the body)?
It’s due to weak adductors. As the knees come in, the adductors are shortened, and thus doesn’t allow them to contract and assist with the lift. So, if you’re adductors are too weak to pull their weight during the squat, the knees will try and come in so the adductors don’t have to work (they can’t contract - they’re already shortened). Mark Rip gave a much better explanation of it than I just did - I looked for the vid on youtube but couldn’t find it.
Hope that kinda makes sense and helps!