Hot tearing defects appear during the last steps of casting solidification of dendrites. The crystalline structure traps a small amount of liquid metal, which is depressed, and cavities are spontaneously initiated. Molecular dynamics (MD) simulations of pure iron are able to predict, on one hand, the high temperature elastic modulus of crystals, and on the other hand, the cavitation event in liquid metal and near the solid-liquid growth front. High values of elastic tensor components are predicted close to the melting temperature. We conclude that a weakening of the mechanical properties of the crystal is not the reason for failure initiation. The crystals are thinned by partial melting as the effect of negative pressure increases. The cavitation is spontaneously initiated under a very high negative pressure. The cavity expands very quickly and applies a pressure wave to the thinned crystals. We propose that this crystal thinning/pressure wave coupling is able to initiate cracks inside the crystal.