I would guess novices and intermediates are most at risk. Novices for obvious reasons. Intermediates due to overconfidence.
I often see intermediate skiers charging along the margins of blue runs, arms out, backseated, thinking they are awesome. Yeah those are the people that cause bad collisions since they are confident enough to go somewhat fast, but not in control enough to avoid unexpected obstacles. At least the newbies aren't generally going very fast, unless they are completely out of control.
According to ski injury.com most collisions occur from someone going fast along the margins of a blue run. Not sure how that factors into overall head injuries though.