If you read the full research on CTE, one of the biggest causes is Sub-Concussive Impacts. Hockey players suffer many sub-concussive impacts in each game. Since there are no symptoms from these impacts, they do nothing to let the brain heal.

The research done at UNC-Chapel Hill shows that football players receive 700 to 900 sub-concussive impacts PER SEASON. These many mild shakings of the brain combine to cause CTE.

fyi, CTE is Chronic Traumatic Encephalopathy, defined as a shrinking or atrophying of the brain.