If the wave climate is dominated by a wave approach in deep water that is very oblique with respect to the shoreline, the littoral drift may render the rectilinear trend of the shoreline unstable. This instability causes large scale shoreline sand waves (L ~ 1-10 km, T~ 1-10 yr) that grow and propagate along the coast, implying erosional hot spots. This was known to be primarily due to gradients in alongshore transport rate. However, it is here shown that the cross-shore transport is also essential to the instability, the faster the relaxation to the equilibrium profile, the stronger the instability. Furthermore, it is found that the cross-shore location of the initial bathymetric perturbation is very important. The maximum efficiency in triggering the instability corresponds to perturbations spread across both the surf and shoaling zones. In contrast, for a perturbation confined only in the surf zone, the development may be so slow that the instability is hardly relevant to coastal engineering. Preliminary implications for beach nourishments are discussed.