We study the spontaneous emission of an excited atom close to an optical
nanofiber and the resulting scattering forces. For a suitably chosen
orientation of the atomic dipole, the spontaneous emission pattern becomes
asymmetric and a resonant Casimir--Polder force parallel to the fiber axis
arises. For a simple model case, we show that the such a lateral force is due
to the interaction of the circularly oscillating atomic dipole moment with its
image inside the material. With the Casimir--Polder energy being constant in
the lateral direction, the predicted lateral force does not derive from a
potential in the usual way. Our results have implications for optical force
measurements on a substrate as well as for laser cooling of atoms in
nanophotonic traps
