You can't run back to back to back without letting a Nx motor cool. The valves will get so hot they will glow, and you risk damaging your heads. Nitrous backfires can be caused by two situations. A nitrous system that is two rich or a system that atomizes the fuel poorly, thus causing pooling or puddling of fuel in the intake manifold. Or a system that is operated too lean.
I would say that you got the motor hot, it started to run VERY lean, a cylinder fired earlier than it should have, and ignited the fuel and nx in your intake. Your VERY lucky that your manifold is in one piece. The other cause could have been puddling, check you nozzle placement. If the nozzle is not down in the airway far enough, or has something in front of it (the center of the TB) the fuel will puddle and not atomize properly and cause the motor to run lean with tons of extra fuel in the intake...