I have not seen the RADAR data, only the report in the Times, but looks to me (an Airbus line pilot) as some are saying: like a deliberate 90deg turn away from the airway followed by an emergency descent, so I am thinking decompression.
Then, halfway down they might have thought 'hang on we are heading towards high ground now', hence the right turn - an orbit or a PPOS hold? - to keep them over the sea while still descending to FL100.
Then something happened. Perhaps the crew oxygen ran out or failed in some way, but the aircraft then appears to have exceeded Vne and broken up?
My thoughts are with them and their families.