This paper investigates why the observed critical Shields stress for incipient sediment motion deviates from the Shields curve. This deviation is often described in the literature as due to measurement errors and non-uniformity of channel-bed slope or flows. On the contrary, this paper hypothesises the deviation as due to the presence of vertical velocity in open channel flows. We found that the existence of vertical velocity in non-uniform flows can lead to the deviation of critical shear stress from the standard Shield's diagram that is valid only in uniform flows, and the channel bed slope could not contribute to the flow's non-uniformity. The new hypothesis was successfully tested with the experimental data available in the literature. In addition, a new theory for critical shear stress has been developed, which shows that the decelerating flows promote the mobility of sediment, but accelerating flows constrain its mobility. A unified critical Shields stress for sediment transport has been established, that can predict the critical shear stress for sediment initial motion in both uniform and non-uniform flows.