  The orientation of the electron-nematic states discovered in the quantum Hall
regime of GaAs $[001]$ growth-direction quantum wells is pinned by a weak
native source of anisotropy. In this Letter we explain that this property,
which has remained mysterious over more than a decade of research, follows from
the presence of both Rashba and Dresselhaus spin-orbit interactions. The hard
transport direction of the nematic state is determined by the relative sign of
the Rashba and Dresselhaus coefficients, and coincides with either the $[110]$
or the $[1\bar{1}0]$ crystallographic direction. Our theoretical estimate of
the pinning energy is in agreement with experimental studies of the competition
between native pinning and intentional pinning by an in-plane magnetic field.
