It is actually not all that uncommon in certain groups. In South Africa's Hluhluwe-iMfolozi Park, 3.7 to 4.0 percent of rhinos face this issue. In South Africa's Mkhuze Game Reserve, that figure rises to 7.1 percent.
The cause of the deafness is unknown. There has been speculation from genetic abnormality to the notion of predator attacks, such as hyena, but no definitive conclusion has been found. In 2012, Roan Plotz, spent three years researching and tracking black rhinos. His observations were that this physical anomaly renders the rhino totally deaf. See more on Plotz's findings here.