Destruction of the underlying cartilage framework of the outer ear (pinnae), usually caused by either infection or trauma, resulting in a thickening of the ear. Classically, blood collects (hematoma) between the ear cartilage and the skin. There is a marked thickening of the entire ear which may be so extensive that the shape of the ear becomes unrecognizable. The ear is said to look like a piece of cauliflower. It is typically seen in wrestlers and boxers who have had repeated trauma to the ear.
When trauma causes a blood clot under the skin of the ear, the clot disrupts the connection of the skin to the ear cartilage. The cartilage has no other blood supply except the overlying skin so, if the skin is separated from the cartilage, the cartilage is deprived of nutrients and dies and the ear cartilage shrivels up to form the classic cauliflower ear.
Source: MedicineNet.com: