The body will always strive (and win) to be balanced, otherwise we fall. If the hips (specifically back hip) fires / rotates just prior to front foot strike, the shoulders will stay back...otherwise the body will be out of balance. Hip rotation prior to shoulder rotation is the ONLY action that will create hip to shoulder separation. Hip to shoulder separation has been surmised to account for up to 80+% of velocity....separation / distance of the back hip from the throwing arm / elbow creates the stored elastic energy (stretched rubber band) ultimately in the arm layback position that creates velocity. Arm action at or inside of 90 degrees at the high cock position aids in this as well.
Edit to add: if you can get the pitcher to learn how to activate the hips prior to FFS, the hip to shoulder separation (keeping shoulders back) becomes a "non-teach" it happens automatically....and to a large degree this is true of arm action. What happens first, directly affects what happens next. Too many instructors teach the symptom, without realizing the cause...this creates IMO - robotic, non-athletic pitchers that all look the same.