Turbo flutter or compressor surge is caused when the throttle body snaps closed causing boosted air to become trapped in the piping between the turbo outlet and throttle body with no where to relieve boost pressure. this causes the pressurized air to escape back through the turbo out the intake.
Most cars come equipped with a blow off valve or a recirculation valve to relieve the pressure back into the intake or into the atmosphere. compressor surge can be harmful to the turbocharger its self but not to the heath of the motor. In the case of a turbo fluttering the air is forcing its way back into the turbo forcing the turbo to slowdown in the opposite direction. the excess force will prematurely wear out the bearings and can even cause the compressor wheel to damage the the inner turbo housing.