OK, when you stand with the weight on your shoulders, did you try to engage your glutes before you squat down? or you didn't do anything just sat down. [that's a huge difference]
I am guessing the latter, your glutes need to be activated before you squat down, and is through the foot, I have press map data to prove this.
see here http://www.secret-of-athleticism.com/pressure-map-data/
When you just stand in the neutral state, your glutes is not activated. you can't just follow the form alone. that's why most people can't do it right.