Wet and dry rot are conditions related to moisture being absorbed and released. Certain materials are more prone to dry/wet rot than other. Rubber, for example, will dry rot over time when left in the elements. The pores allow it to absorb and release moisture, and that process removes/breaks down some of the fibrous materials that make up the rubber over time. There are many materials subject to this process other than rubber, but I am not going to provide a comprehensive list. More or less, anything that can absorb moisture will rot over time if left in the elements.