Looks like you really drop at the start of your stride. (I assume that's what you meant by collapsing your back leg.) That represents wasted energy because once you collapse the leg you then have to exert force down toward the ground to halt that downward movement. Energy directed toward the ground is energy NOT directed at the target.
Also, your stride looks like a big step. That is, it looks like you take a big step with your stride foot. I'd prefer to see you lead with the front hip a little longer into your stride before swinging the stride foot out front. That will create more momentum resulting in more energy to put into the ball as well as a longer stride. It will also set up a better load for forward trunk flexion.