Bagwell sits very deep in the chair, he places his weight on and against the back leg (as he said while motioning with both hands on the side of his torso). With the wide stance, and the butt to far behind and to low, the body adn torso must unload somehow. It must rise a little, the front foot comes off the ground during this time and is set back down (moving back a few inches).
Combine the deep chair and hand action slightly down when he is already so deep, when the hands raise a little so does the body or center of gravity so it can unload. Thus the front foot moves slightly back.
I don't like people using Bagwell as an example as "how important is the stride if his moves backwards". How important can momentum be if he strides backwards?
I've heard it 100's of times, without discussing why it moves backwards and what is happening in his swing. No one has given an analysis on his swing as to what is actually happening, or at least what they think is happening.
It is more of an argument for "what is the stride important for" instead of "how important is the stride."