What caused the fire to burn (chemically) at the Triangle Shirtwaist

Fire in 1911?

Court testimony later placed the blame for the blaze on a fire that started in a fabric scrap bin on the eighth floor, which probably was ignited by a discarded cigarette, shortly before the factory's 4 pm closing time.

In short they believe a cigarette butt was dropped on a piece of highly flammable fabric even though cigarettes weren't allowed.