I think Ferdoruk's quote explains a lot. You likely have to be a certain type of person to accept the role of being an NHL enforcer, especially today when the majority of fighting is placed on one or two players per team. However, I don't recall seeing these issues in the late 80's and early 90's when many of the league's most active fighters were playing/retiring.
Yes, there is a pension for players who play more than 400 games.
Belak was going to be working as a broadcaster for the Predators and was in Toronto training for Battle of the Blades on CBC. He was one of the lucky ones who had opportunities post-hockey to make a living.
I don't think money is a factor. I think getting punched in the head hundreds of times with bare knuckles and the cumulative effect it has on the brain is more at play here, but that's just my opinion.
Until there is an actual link to fighting, and not the personal demons these players had, I think naming this as the cause is a bit premature (as you suggested). I hope this is just a terrible anomaly that will help other players who are battling personal demons.