Occipital bun is a prominent bulge, or projection, of the occipital bone at the back of the skull. Occipital buns are important in scientific descriptions of classic Neanderthal crania. While common among many of humankind's ancestors, primarily robust relatives rather than gracile, the protrusion is relatively rare in modern Homo sapiens.
Some scientists suspect occipital buns might correlate with the biomechanics of running. Another theory attributes them to enlargement of the cerebellum, a region of the brain which mediates the timing of motor actions and spatial reasoning.
There are still many human populations which often exhibit occipital buns. A greater proportion of early modern Europeans had them, but prominent occipital buns even among Europeans are now relatively infrequent.
A study conducted by Lieberman, Pearson and Mowbray provides evidence that individuals with narrow heads (dolicocephalic) or narrow cranial bases and relatively large brains are more likely to have occipital buns as a means of resolving a spatial packing problem. 
- PBS.org - 'Neanderthals on Trial' (January 22, 2002)