But it also makes a lot of sense. Nitroglycerine, both in the game and in real life, is a very unstable liquid due to its heavy molecular structure. So what I THINK happens is that the third shot causes a small amount of the stuff to release a strong pressure wave. Long story short, a shock wave travels through the NG at about mach 30(30 times the speed of sound), causing a nearly-instantaneous pressure-induced decomposition process, converting it into a white-hot gas(about 9,030 °F/5,000 °C), which eventually occupies 1,200 times the original volume(and this is only at average room temperature and pressure!). So, there's your explanation, and I hope you learned something.