Clues Emerge in SpaceShipTwo Accident
The lock/unlock lever that holds the feather in position on SpaceShipTwo was manually moved into the unlocked position by the co-pilot during the powered phase of flight. The first of two steps required to unlock the feather were initiated, however the second step of actually moving the feather position was not taken. Chairman Hart of the NTSB referred to the actual movement of the feather as an "un-commanded" move.
The feather provides drag to slow the spacecraft and also orient that craft nose down for the proper reentry profile - working much like a badminton shuttlecock.
The NTSB video offers further details - as do the photos diagrams of the feather sequence.