This work explored the mechanism of spontaneous combustion of hydrogen-oxygen mixtures inside nanobubbles (which were generated by water electrolysis) using reactive molecular dynamic simulations based on the first-principles derived reactive force field ReaxFF. The effects of surface-assisted dissociation of H-2 and O-2 gases that produced H and O radicals were examined. Additionally, the ignition outcome and species evolution as a function of the initial system pressure (or bubble size) were studied. A significant amount of hydrogen peroxide (H2O2), 6-140 times water (H2O), was observed in the combustion products. This was attributed to the low-temperature (similar to 300 K) and high-pressure (2-80 atm) conditions at which the chemical reactions were taking place. In addition, the rate of consumption of H-2 and O-2 molecules was found to increase with an increase in added H and O radical concentrations and initial system pressure. The rate at which heat was being lost from the combustion chamber (nanobubbles) was also compared to the rate at which heat was being released from the chemical reactions. Only a slight rise in the reaction temperature was observed (similar to 68 K), signifying that, at such small scales, heat losses dominate. The resulting chemistry was quite different from macroscopic combustion, which usually takes place at a much higher temperatures of above 1000 K.