The NTSB investigation concluded that the crash was caused by the pilot steering the plane down. However, the FBI criminal investigation found that that pilot didn't seem to fit the profile of someone suicidal.
Therein lies the hugely important puzzle which I don't think has received enough attention: it seems there are some extremely major holes in our understanding of psychology regarding suicide, things that, if understood, among other things (many other things), would be helpful in increasing air passenger safety.
We wildly speculate a mechanism:
Suicide occasionally pops into certain people's minds like a compelling itch to scratch, not consciously controllable and not obeying rational thoughts. (Pretty frightening: anyone, including you, might suddenly off themselves one day and have no control over it.)
Certain risk factors may magnify whether thoughts of suicide turn into suicide carried out.
The speculated mechanism is infection by a microorganism, inspired by suicidal behavior in mice caused by Toxoplasma. Our speculated microorganism also somehow benefits from its host dying, or at least did over the course of its parasitic relationship with the human species. For example, it must spend part of its life cycle in soil, assuming burial of the dead has been common.
Any biological explanation of suicide must explain how it has persisted through evolution. The reason humans haven't evolved resistance against this hypothetical pathogen is because the it taps into the natural ability of the brain to be intensely curious, to obsess, over something. This capability as a component of intelligence has been generally evolutionarily beneficial. The microorganism just slightly modifies that neurological pathway to make the brain obsess about death.
The effects of the infection (suicidal behavior) might also only occur later in life when the immune system has weakened, after children have been conceived, thereby avoiding evolution.
(Previously about parasites and evolution.)
Inspired by Germanwings 9525.