It looks to me that you’re kind of stalling out on your back leg (squatting) rather than trying to go down and out at the same time. Basically you should be moving out on your back leg down the mound more quickly rather than going straight down as your first move. It also looks like you also swing your stride leg back causing you to lose momentum forward–it acts as a counterbalance for you to cause you to shift weight backward instead of keep moving forward faster. This seems to be hurting your hip/shoulder separation, as well as causing you to get your arm up very quickly (your arm looks like it is cocked and ready to throw very early). I would try to increase the speed that you’re trying to travel down the mound. Also try to break your hands later by getting momentum with your throwing arm out of your glove (hands in front like you’re praying, and develop a tempo and rhythm to break your hands down in front of you).
This is a model of Nolan Ryan throwing for example (watch his hand break and the way he moves down the mound):
“Once Ryan’s hands start to separate, he captures and maintains that energy bringing the ball and glove down and then up again in one continuous motion, as a roller coaster would go down the track and then up again capturing much of its down track momentum.” [Nyman]
Your arm during your stride (notice how little you’ve moved forward/down the mound at that point):