Your head and feet are moving at the same speed. Very fast. The board leaves you behind by stopping. You now put one foot on the ground. Because of friction, your foot stops and goes flying behind you before you can put your next foot on the ground or apply any meaningful weight to it. Because your foot was thrown backwards, it applies angular momentum to your body and you begin spinning around your center of mass (aka, falling forward). Less than 100ms has passed at this point. In the best case, your other foot is impacting the ground, right now, directly under you. But it can't take your weight before it has flung behind you and adding to your angular momentum, forcing you to go face first. It is at this point, you are capable of realizing you are going to fall face first. You have ~80ms to realize this, and put your arms up, before impact. Even if you have amazing reflexes, an impulse isn't reaching your muscles in less than 40ms, that means you need to move your arms from 'keeping balance mode' to 'defensive mode' (including applying resistive power to keep them there) in less than another 40ms. It literally isn't happening.

People who fall from standing by their own volition usually have a sense something is wrong and can fall relatively gracefully. People who are tripped from standing collapse like a sack of bricks. Same thing, you don't see falls coming on these things.