The research tends to point toward extreme numbers of subconcussive impacts as the likely primary cause of CTE. They found that individual football players usually suffered 800 or more subconcussive blows in a single season. They have observed CTE is players with no record of concussions but who played positions that were common to cause high numbers of subconcussive impacts.

They don't understand why some who have those high numbers of subconcussive impacts do not develop CTE. There is a thought that there may be a genetic component at play.

There is a strong reason to not live a life in anxiety of CTE. Anxiety reduces coordination making it more likely to suffer a concussion.

Normal risk reduction decisions like no contact sports or other risky activities and good brain health hygiene then getting on with life is best. Wearing a helmet in some activities (skiing, snowboarding, etc) is worthwhile.