Who is at fault if Driver B was changing lanes and then Driver A rear ends Driver B and then Driver B leaves the scene of the accident?

The fact driver B left the scene does not change that driver A is At Fault (there are exceptions to driver A being at fault)

Additional: While Driver A may have been the striking vehicle, Driver B MAY be at fault for "Changing Lanes Without Caution." Regardless of the circumstances of the collision itself, Driver B can be charged with "Leaving the Scene of an Accident."