Any defects or impurities within or at the surface of the semiconductor promote recombination. Since the surface of the solar cell represents a severe disruption of the crystal lattice, the surfaces of the solar cell are a site of particularly high recombination.
The high recombination rate in the vicinity of a surface depletes this region of minority carriers.
As explained in the Diffusion Page, a localized region of low carrier concentration causes carriers to flow into this region from the surrounding, higher concentration regions.