In this study we have examined the combustion dynamics of 1,5-dinitrobiuret (DNB) and nitric acid using reactive molecular dynamics simulations. Simulations were performed using the ReaxFF force field with parameters that were fitted against quantum mechanical calculations on model compounds/clusters relevant for this particular chemical system. Several different compositions were investigated, at densities of 0.5 and 1.0 g/mL, to examine the reaction kinetics in a dense vapor and liquid phase of these mixtures. Our simulations show that at certain compositions of the mixture reaction kinetics result in a very sharp release of thermal energy, which we associate with spontaneous ignition or hypergolicity. Analysis of key reaction mechanisms responsible for this process is discussed.