Cotton and linen can ignite when they come into contact with polyunsaturated vegetable oils (linseed, massage oils); bacteria slowly decompose the materials, producing heat. If these materials are stored in a way so the heat cannot escape, the heat buildup increases the rate of decomposition and thus the rate of heat buildup increases.
Certain fabrics like cotton or linen can catch fire when they come into contact with specific oils, such as linseed oil. Meanwhile, bacteria slowly break down these materials, producing heat. If stored in a way that traps this heat, the decomposition process speeds up, resulting in faster heat generation and increased breakdown.