Medici,
You may be trying to get the club right back using your pivot, i.e. you are over pivoting...
If you stand up straight and turn your head to the left, you will find your head will only turn about 70-80 degrees to the left. If you try and turn it any more it will push your left shoulder around to the left also...
Correlating that to your golf swing, unless you turn your head to the right before you start the takeaway ( a la Jack), at the 70 degree point your shoulder will push your head around (to try and get your full turn)..this can cause you to straighten up.
A better way to do it is to turn the shoulder to its maximum (70-80 degrees in relation to your head) and the rest of the "90 degree" turn is achieved by the movement of the shoulder blades...i.e. you add abduction (extension away from the spine on a horizontal plane if you were standing upright) to the left shouder blade, and adduction (movement towards the spine) of the right shoulder blade...
Personally I only think about the right shoulder blade at the top, i.e. I compress it towards the spine as much as I can....the left shoulder then takes care of itself...
