Can sports head injuries/concussions be fatal years later?

Not usually. Severe head injuries such as concussions can have long term effects (memory loss, dementia, mood swings, change in personality, psychiatric problems) but aren't usually directly associated with a fatal outcome years later.
Another view. There are professional athletes with a history of head injury that have been documented to experience significant mental decline, depression, headaches and suicide. Those examined by pathologists post mortem have been diagnosed with cte (chronic traumatic encephalopathy). Look up the legacy institute for the latest info.
Very Possibly. We are learning that concussions are cumulative. The more we get the more damage there can be later. Changes in the brain are similar to some other neurological diseases. We know that the more concussions we get, the more susceptible to the next one we become. There is ongoing research looking at cumulative concussions in ex nfl players. Google it. You'll be able to read a lot about it.