Looks like a plenum leak,prob blew the enrichment fuel charge out and ignited it.....Fook all to do with the Nos on its own cos Nos don`t burn,it just liberates loads of oxygen to assist the combustion process......

Someones been watching too much Fast and Furious,the only thing dangerous about Nos is the fact that it`s compressed gas,It doesn`t explode in free air and it certainly doesn`t blow the welds on the intake.......