Leeson13,
Your Back Leg collapse causes your Front Leg to open like a gate and, at the same time, prematurely open your Hips. At Foot Plant, because you've already lost your Hips, you throw mostly Upper Body. Without your Lower Body connecting to your Upper Body, your velocity and command suffers.
To correct this, you must change your Front Leg position in your Starting Position so you begin your motion athletically stacked (with your weight centered within your Body).
A stacked Starting Position makes your entire motion more dynamic, will allow your Throwing Hand to become more of an extension of your Hips and allow you to realize better command/velocity.
L.A. "Skip" Fast
Pro Pitching Institute