Yes, that is indeed a giant fireball erupting inside the Superdome. You will see a kickass fireworks show if you go to any of the remaining MESS supercross races (and I highly recommend that you do). American supercross has always had a decent display of detonations, ever since the Pace/CCE/LiveNation and now FMS folks have run the show. Of COURSE the show producers are setting off fireworks, pulling off successful stadium shows for upwards of 40,000 paying customers requires those same producers to create a memorable spectacle, no matter what happens on the track.
And, yes indeed, I used the word "show" four times in the preceding paragraph. For almost as long as "supercross" has existed in the USA (30 yrs?), series achieved it's largest successes at the box office when marketed as a piece of theater, grand entertainment for the throng of willing American consumers. That's undoubtedly one of the reasons behind the decision to create FMS and buy the series.
"But it's a serious motorsport! The dynamics of supercross are natually "explosive", with racing machines blasting out of tight turns and launching over huge triples; man-made explosives are not necessary. Why can't the promoters conduct it as a serious motorsports competition (think events like the 24 hours of Le Mans or the Formula 1 series) one worthy of the respect of the world?" my naive inner motogeek asketh outloud. There will be no satis-factory answer, just go and enjoy the show. It could be worse.