Ruling out terrorism is a great idea, as the point of terrorism is to say "Here weare! The ____ has just cuased massive loss of lives and thousands of dollars worth of damages! And we'll do it again! Hear us roar! You shall succumb to ____!"
And most of that doesn't apply. The thousands does, but... it's kinda just a plane, not any sort of building, which seems unusual. Granted, it's still likely in that regard. But the fact that no one knows who it was that tells us. One of their points to let their target know who's done the deed. It's to make themselves more ferocious. Yet... none of that. So it's probably not terrorists.
My guess?
Uh... well, probably some guy got pissed off/ a lust for blood and crashed the plane. It's not a very good guess. Another could be a possible plane malfunction that caused the crash. Let's say the pilot got sleepy and decided to let the plane be on autopilot. And there was a small glitch that sent them a few degrees in the wrong direction. That's my guesses.