In the mid-20th century film switched to modern plastics like cellulose acetate, and later PET (which are still quite flammable). But before that, it was the first kind of plastic invented -- nitrocellulose plasticized with camphor. Nitrocellulose is also known as gun cotton; it's literally explosive.

It degrades over time, with the plasticizer separating and the film becoming brittle, eventually turning into a powder that can ignite from friction or even auto-ignite. More than anything else, that's why so many early major films are lost. They turned to dust. And then exploded.

> The use of volatile nitrocellulose film for motion pictures led many cinemas to fireproof their projection rooms with wall coverings made of asbestos. Those additions intended to prevent or at least delay the migration of flames beyond the projection areas. A training film for projectionists included footage of a controlled ignition of a reel of nitrate film, which continued to burn even when fully submerged in water. [...] Owing to public safety precautions, London Underground forbade transport of movies on its system until well past the introduction of safety film.

https://en.wikipedia.org/wiki/Nitrocellulose#Nitrate_film_fi...