Co-pilot acted alone
People familiar with the NTSB investigation say Alsbury unlocked the tail mechanism of SpaceShipTwo without authorization from Siebold, and two seconds later the spaceship disintegrated. This is said to have caused the tail-feathering mechanism to become loose and create turbulence that buffeted the spaceship apart, as airspeed was 250 miles an hour slower than the rate needed for airflow to hold the tail mechanism in place while unlocked. Will Whitehorn, who served as president of Virgin Galactic until 2011 says it remains a mystery why Alsbury unlocked the tail without conferring with Siebold:
Nobody knows at this stage.