Hip flexors and squatting

I have a question that I hope that some of you can clear up for me. I have read many articles and discussiond regarding indivual limitations with the squatting motion. In many of these, lack of ankle mobility and hip flexor flexibility are cited as contributing factors to limitation in ROM. I can see where lacking adequete dorsiflexion of the ankle joint can be limiting, but am not quite following where tight hip flexors would hinder progress. During descent, hip flexion is occuring and is eccentrically controlled by the hip extensors. I know that with the rectus femoris, it is lengthening at the knee joint, but is still shortening at the hip joint which would compensate for a passive insufficency. I am probably thinking about this too hard and my methods of research class is eating my brain, so there may be a very simple explanation that I am overlooking. Any help or feedback would be greatly appreciated.

I answered this last night on your other post.