basically nitrous is air....dense air. So when you get on the throttle, before the charger is lit up, you are dumping unburnt fuel out of the tail pipe....if the exhaust temps are hot enough, and you have the excess fuel in the exhaust, and too much nitrous.....then boom you get combustion in the exhaust.
That is why mine is set up with a little jet for the first stage (.018), which is on a WOT switch....it will help light the charger, then the 2nd stage with the .032 jet is button fired....i press the button as soon as i see 25psi of boost.