It was caused by a spark from some grinding machines with whose aid workers were in the process of attaching an iron curtain. Precisely this was supposed to prevent the flames of any potential fire from spreading beyond the stage and causing havoc among the audience.
Ocurrió a raíz de unas chispas que saltaron del soplete de unos operarios que en aquellos momentos reparaban el telón de acero que debía impedir que, en caso de incendio, las llamas llegaran del escenario a la sala.