[Impy]

Do we know for a fact Seau’s suicide had anything to do with concussions?

[John W]

Not yet.

[cripplecreek]

Do we know for a fact Seau’s suicide had anything to do with concussions?

Nope, and we never will.

[x]

We don't know why Seau did it, and may never know with certainty. There will be an autopsy to determine if his brain was damaged.

Wikipedia says:

Though Seau "was never listed on an NFL injury report as having had a concussion", Seau's ex-wife Gina told the Associated Press after Seau's death that Seau sustained concussions during his career. "'Of course he had [sustained them]. He always bounced back and kept on playing.'" she said.

But if it wasn't his death that put this in the spotlight it would have been somebody else's.