It’s very fortunate that it has not “exploded” (self combusted). Many old films are made with nitrocellulose. They can become unstable and just catch fire. They pop quite a bit when they are burning. It sounds a bit like gunfire. I had that happen with some old newsreels that I used to own.
There are still some things made of nitrocellulose and they all love to burn. Ping pong balls and guitar picks come to mind. Back in the day they made combs, eyeglass frames and all sorts of things from it.