Just for info... To set ANFO off you need an initiation with a shock wave.
Could an exploding firework provide that shock wave? Maybe but not likely IMO.
Back in the day we used a stick of dynamite inside a bag of ANFO to set it off. They were 20 kg bags and we used one stick per bag. They were set off with Det cord and a blasting cap. And our farmers use ANFO to lift tree stumps when clearing land.
Ammonium nitrate is mixed with Diesel at a specific ratio to make ANFO. If you get that ratio wrong it wont be explosive. You can also add powdered aluminium to "boost" it.
https://en.wikipedia.org/wiki/ANFO
IMO We'll never be told the truth of what happened in this instance.