Why is plane-bed topography unstable under certain flow conditions? We investigate the grain-scale mechanisms responsible for topographic instability at the onset of bedform development. Measurements of fluorescent tracer particle motion were used to estimate the ensemble mean particle activity, entrainment rate, hop distance, travel time, and particle velocity characteristic of flow conditions straddling the threshold stress for bedform development. Based on these data, we propose two hypotheses to explain the destabilization of planar topography with rising transport conditions. Hypothesis 1: plane-bed topography is unstable above a theory-predicted entrainment rate threshold that varies primarily as a function of particle diameter. Hypothesis 2: plane-bed topography is unstable above a threshold particle collision frequency that is proportional to bedload flux.The threshold particle collision frequency is predicted analogously to the propensity for congestion shockwaves in vehicular traffic flow theory.