I think it's simply muscle memory. They are conditioned to throw a certain way for a living. Having to deviate from that, especially so quickly, is hard for anyone. I think if anybody had to reptitively perform an action that required skill over and over then suddenly had to switch up the form on that, on the fly, without much warning, would screw up on a regular basis. Not an excuse but just an explanation. It's why so many pitchers will toss the ball to 1B underhand if they have time.
Looking at that play again... did the runner touch the plate? If he had made a good throw it could have been a hell of an ending the other way because he beat the ball no doubt, but if he missed the plate and they went to review....