most logical explanation so far
the totality of what we know at this point IMHO, points to deliberate "hijacking" of the plane by either the captain or copilot. an external hijacker is not going to be savvy or technically proficient to fly through these navigational waypoints, in addition the pilot apparently timed the flight diversion to after the plane left the first country's air traffic control etc.
I suspect this will turn out to be suicide by pilot, I believe for whatever reason he was trying to hide his plot in this odd way. if they haven't already investigated thoroughly the pilots personal life's, financial situation, evidence of depression etc that would be a serious oversight.