The elastic moduli of ice VIII at different temperatures and pressures have been calculated from the quasiharmonic lattice dynamics method employing the TIP4P potential for water. It was found that under decompression, one of the Born's stability conditions for solids was violated and ice VIII became mechanically unstable which led to a phase transformation. The transition pressure was found to decrease with temperature. This phenomenon is a symmetric equivalent of the pressure-induced crystal --> amorphous transformation in ice Ih. Based on the theoretical results, it is proposed that the observed transformation of ice VIII to high density amorphous ice at low temperature is probably due to a mechanical instability in the crystal.