We report polar instability in molecular materials. Polarization-induced explosive decomposition in molecular crystals is explored with an illustrative example of two crystalline polymorphs of WAX, an important energetic material. We establish that the presence of a polar surface in (delta-HMX has fundamental implications for material stability and overall chemical behavior. A comparative quantum-chemical analysis of major decomposition mechanisms in polar delta-HMX and nonpolar beta-HMX discovered a dramatic difference in dominating dissociation reactions, activation barriers, and reaction rates. The presence of charge on the polar delta-HMX surface alters chemical mechanisms and effectively triggers decomposition simultaneously through several channels with significantly reduced activation barriers. This results in much faster decomposition chemistry and in higher chemical reactivity of delta-HMX phase relatively to beta-HMX phase. We predict decomposition mechanisms and their activation barriers in condensed delta-HMX phase, sensitivity of which happens to be comparable to primary explosives. We suggest that the observed trend among polymorphs is a manifestation of polar instability phenomena, and hence similar processes are likely to take place in all polar molecular crystals.