High squat mobility, poor control. This is more common in females, but also in males who've played sports that demand extra mobility as a kid, or who have a lot of congenital joint laxity.
For these people, they need to focus on trying to control the movement more, which means slowing down and thinking about positioning and trying not to bounce through the movement.
Ever had one of those days where you just couldn't get into the flow of your workout? Or worse yet, you don't even make it to the gym? It's happened to all of us before. The mind is willing, but the body just isn't ready (or vice versa).