Previous research has shown that a very oblique wave incidence on a coast may render a rectilinear shoreline unstable. Here, we present some further insight into the physics of such instability. The obliqueness of wave incidence has two effects on the alongshore drift: (i) a direct effect on the relative angle between the wave fronts and the shoreline, and (ii) an indirect effect on the breakers height via the wave energy spreading as the waves refract when they approach the shore. The direct effect turns out to be, in all incidents, stabilizing, and the instability occurs only from the effects of the wave energy spreading, which dominate for large incidence angles. Whereas earlier studies have pointed only to the alongshore drift as the cause of the instability, we show that the instability mechanism involves both the surf and the shoaling zones, so that the link provided by the cross-shore sediment transport is also essential.