From my own experience in the gym, leaning forward can cause an almost instinctual movement to lift the bar by "pushing back" with the back.

That is, rather than lifting the bar using the legs, lifting the bar by bending back. Of course, Rippletoe would never recommend doing this, and while leaning forward doesn't guarantee improper form, it can sure make it easier.

Thank you for clarifying that. That is exactly what I meant but obviously failed at communicating.

Leaning forward is okay while doing squats. A heavy weight causing you to lean forward out of your position is bad.