Spontaneous combustion of sulfur in a steel barrel
Iron sulfide FeS is pyrophoric: flammable, and while it can start just on exposure to air, the presence of UV light will help.  As is usual, one of the key components is the surface to volume ratio.  A massive lump is unlikely to catch fire.  A crusty scale of FeS, such as forms in tanks used to store sour oil (H2S content) is more likely to combust when air is let into the tank.

Deactivation of Pyrophoric Iron Sulfides

From the descriptions in the abstract above, I suspect that I had one of the other iron sulfides.  Whatever I had did not burn fast.