Half of all American films made before 1950 and over 90% of films made before 1929 are lost forever. The vast majority (75 to 90%) of silent films are lost forever. This is mostly due to fire caused by the nitrate used in the film. Projectionists used to have an escape tunnel from some projection rooms in case of fire.
I don't think fires during projection were responsible for most lost films. If one or two prints of a particular movie were lost that way, surely others would have survived. Rather, Hollywood just didn't care to keep most such films around; many were tossed, reused (nitrate base stripped of the emulsion and recoated), or relegated to poorly run vaults in which they slowly degraded.
