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Abstract Over the past seven years, the capsizing of considerable bulk carriers with nickel ores in the South China Sea has made shipping industry shocked. Although a number of countermeasures have been taken by ship owners, it doesn't seem to prevent the ship accidents from happening, possibly implying that we may not yet fully understand some fundamental aspects of the accidents. Clay lateritic nickel ore is characterized by liquefaction under the influence of waves or long-time engine vibrations during transportation. However, the existing methods based on the calculation of GM (metacentric height) values fail in explaining the capsizing of the bulk carriers during shipment. Experimental investigation into the accidents shows that the blending liquid (not pure water) after liquefied is highly viscous and can generate free surface. In contrast to the prevailing theories of ship stability, this free surface may create negative influence on ship's stability, and sometimes may result in a severe consequence of capsizing. Therefore, we speculate that the viscosity coefficient may play a critical role in ship's capsizing. Here we take advantage of numerical simulations to make a thorough analysis on the physical mechanisms of capsizing. The conclusion has been drawn that the collapse of the shear force which is mainly determined by the viscosity coefficient has led to a shift of the liquefied cargo when the inertial force of the liquefied cargo is greater than the shear stress, thus causing an abrupt loss of the stability. Finally, the increasing external moments result in capsizing and sinking. This finding may be profound because it reveals a substantial relationship between the capsizing of a ship and the viscosity of the liquefied cargo which has never been described in previous literature. Highlights • Liquefaction of the nickel ores during shipment has led to numerous capsizing and sinking. • The collapse of the shear stress force may play a critical role. • Numerical simulations have demonstrated that the sloshing of the cargo after liquefied increases the heeling moments. • The collapse of the shear stress force is significantly associated with the frequency and amplitude of the ship rolling. [ABSTRACT FROM AUTHOR]