I don't think we do know. There were just some suppositions and then judgements early in the thread. Some people jumped on the judgements. I jumped on what I perceive as a dangerous myth about people dying in cold water in under 2 minutes due to hypothermia.
In the end, I think this is really a we're sad at the loss of a "brother" and a warning that cold spring rivers and specifically the Kennebec River can be dangerous and often underestimated in their danger.
Everything else is pretty much drift.