What was the major cause of those players with early stages of CTE?

The major cause of Chronic Traumatic Encephalopathy (CTE) in athletes with early stages of the condition is repetitive head trauma, typically from participating in contact sports such as American football, boxing, or hockey. These repetitive blows to the head can lead to the accumulation of abnormal proteins in the brain, causing degenerative changes and eventual neurological symptoms associated with CTE.