What causes stall buffeting?
This article states it pretty well (better than I can word it). 


  When the angle of attack (AOA) of the wing increases, the point where
  the airflow separates will move forward and the streamlined airflow
  will become turbulent and separate from the wing. This turbulent wake
  then meets the aft fuselage and tail section of the aircraft. This
  will be felt by the occupants of the aircraft as a rumble or buffet.
  Not all aircraft have a pronounced buffet, this depends on the size
  and location of elevator.


Interestingly enough you can also induce a stall (and first high speed buffeting) by gong too high and too fast while trying to maintain level flight which will cause the center of lift to move aft along the wing. Basically your stall speed and your critical mach number eventually converge this is known as the coffin corner. There is a nice discussion on it here.