Maybe the aggressive compressions of slalom cause the brain to bounce around a bit? In any case, I'm sure it's much less than the sudden stops caused by running into a 300 pound lineman.
The Earth weighs a good deal more than a 300lb lineman, and it's the Earth that skiiers are repeatedly running into while carrying an enormous amount of inertia.
Have you ever been skiing? It's your feet that are running into the earth, and hardly even that - they're skating smoothly over it unless you really mess up. On good mogul run,your head hardly moves off a straight line while your legs and hips adjust to keep your skis moving smoothly. In a similar comparison, walking does not produce head trauma.
I've been skiing plenty of times. Runs where I'm at a high rate of speed and hitting countless variation changes (not moguls, just normal skiing) and my head is shaking so much that my vision is blurry. Landing a jump at best (when you absorb with your legs) is a 19G+ event. Every minor mistake in absorbing can tenfold increase that. Walking is not a relevant comparison.
We evolved to walk with a limited brain suspension that just wasn't adapted for 100kph wax shoe runs.
That's the forward component of the velocity vs the component that's normal to the ground, though - the collision with the Earth is much lower speed than the lateral motion (sliding along the surface after the fall). If you're skiing aggressively, you're frequently quite close to the ground, and a fall is frequently where you end up grazing it because you lean further than the centripital force can maintain. You fall a much shorter distance toward the Earth than a timid skier does.
If you're skiing at the level I'm talking about, falls are probably not that frequent, though.
In my earlier comment I was talking more about the explosive vertical compression-expansion cycle that characterizes skiing really hard with very tight turns on short radius skis. Those are much less violent than hitting a lineman though.