Clearly, WMB has trouble throwing that way. It does not matter who was successful throwing that way...he is not. All you had to do was watch the games last year.
As for why, from what I can see, it almost looks like he is throwing a bit of a sidearm screwball which is why I think the ball moves. It even dies at the end something like a screwball. While he rarely overthrows, more often coming up short, when he does overthrow his wrist and hand are a bit more behind and under the ball. While rare, his overthrow really looks awkward as he is releasing the ball. I suspect when he overthrows he has opened up to early and his arm ends up too far away from his body, almost pushing the ball toward 1st. But that throw is rare compared to the dying quail throw.
His underthrow is a very difficult scoop for the 1st baseman cause' the ball is not likely to bounce true very often. Hitting the ground should slow it up more than a ball thrown over the top that is short, but it will rarely bounce true.
So I would definitely agree that his overthrow is a matter of footwork. I can't really attribute the action he puts on his sidearm throw to footwork. Maybe his grip is doing it. I can't see his grip on that sidearm throw. I can't say that I notice anything about his grip when he does come over the top as the ball does seem to come out true when he throws that way.
Myself I have rarely seen guys throw sidearm from 3rd...3/4's often, but not sidearm unless charging the ball with no alternative.