The steady shear of weak colloidal gels results in vorticity aligned density fluctuations. These have been measured in neutron scattering and flow dichroism experiments and observed with microscopy coupled with rheometer tools of varying geometry. The origins of this instability remain a mystery, and discrete element simulations of colloidal gels have to date failed to reproduce the phenomena. Novel Brownian Dynamics simulations with hydrodynamic interactions show that this instability is fluid mechanical in origin, and results from long-ranged hydrodynamic interactions, which stabilize the vorticity aligned flocs under flow. Squeeze flows between vorticity aligned flocs prevent collisions and realignment under flow, thus promoting stability of large-scale, vorticity aligned density fluctuations. A single force scale-the most probable rupture force for the intercolloid bonds-collapses the microstructural and rheological data, including the characteristic size of the vorticity aligned flocs and the virial contribution to the relative viscosity of the dispersions, across different shear rates and strengths of interaction. The results are presented in terms of a rescaled Mason number, Mn*, describing the ratio of the strength of shear flow to the most probable rupture force, and two distinct regimes of the shear response critical to both computational and experimental studies are identified: dynamic yield and steady-shear flow. The nonlinear rheology in sheared colloidal gels and measures of their structural anisotropy seen in simulations agree well with a wide variety of experiments and are independent of the regime of steady-state response. The study concludes with remarks on the relevant consequences for future computer simulations and experiments on sheared attractive dispersions. (C) 2018 The Society of Rheology.