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.
This situation occurs when cotton, linen, or other cellulose-based materials come into contact with certain oils; meanwhile, bacteria cause them to break down over time, producing heat. If these materials are stored in a way that prevents the heat from escaping, this process accelerates, resulting in faster breakdown and increased heat generation.