Piles of hay, charcoal, wood chips, cotton, and even paper will sometimes spontaneously burst into flame. This isn't because they're too dry. It's because they were stacked up when they were far too wet.
Spontaneous combustion, or a sudden eruption of fire, sounds a lot more mysterious than it is. It gives the impression of being a nearly supernatural event that can't be stopped. Actually, it's so common that many industries take pains to prevent it from happening all the time. There are barns, hay fields, forests, compost heaps, and, once, a two-ton pile of wood chips that have spontaneously caught fire.
And really, water is the culprit. Water makes biological processes possible. And biological processes can generate a great deal of heat.
It starts with respiration. Plant cells can take quite some time to die, and if there's a lot of water inside a plant - if it's too green - the plant will breathe, putting out heat and "sweating" out water. Then the bacteria and fungi take over. If they have a warm, wet, home, they'll munch on the plant matter around them, generating even more heat, and reproducing. Water even plays a part in spreading heat, steaming outwards and warming the area around the center of the pile.
Eventually the temperature hits a critical point, and the pile begins to smolder. This is especially a problem in materials like straw or peat, that form natural kindling that ignites with tiny points of high-heat and little spare oxygen. More terrifyingly, the pile can heat and heat, but without exposure to oxygen can't actually start to burn - until someone rakes into it and exposes the super-heated material to air. Then it bursts into flame from the inside out.
Even metal can help water turn a pile of plant matter into an inferno. Some metals, like copper, effectively lower the combustion temperature of the material around them. They act as catalysts, grabbing oxygen from the air and releasing it to the material, letting it start to burn easily. In the case of the two-ton woodchip pile, water diffused outwards, heating wood that had been covered in acetone before it had been ground up. Iron salts in the pile heated up enough to set the acetone alight, and the entire thing went up in flames. Sometimes, water is exactly what you need to get a fire going.