As the set goes on, there is a timing disconnect between your hips and knees breaking. It is very apparent by rep 5. So, what happens when your knees are held back early? They have to move forward at the bottom so that you stay in balance. Cue "knees" and try really hard to get them in place earlier.
Also, you might stand to move your stance out just a bit. This may help you keep your knees from moving any more, provided you shove them out hard enough.