The leaves crack. Too much water in the plant tissues result in increased turgor pressure; eventually the pressure breaks the cell wall. Normally, transpiration (loss of water through the leaves) alleviates the pressure coming from the contents of the cells. The leaves of most succulents, however, are designed to hold water. Transpiration, which is a constant event in most plants, balances the water level in the cells. In the case of succulents this process is set to very low level or even none at all. Sometimes they have leathery or waxy leaves from a thick layer of epidermis that keep the water inside. This is how they adapt to dry environments. However, when they are exposed to conditions of excessive water their defensive virtues work against them.
Not all succulents respond to excess water in the same way. Some of them rot starting from the parts that are directly in contact with the excess moisture.