There are several medical conditions which cause a loss of censorship of speech. Tourette Syndrome is a brain condition that, among other symptoms, causes the patient to curse uncontrollably. Schizophrenia, a mental disorder that is characterized by misperceptions of reality, can cause people to act violently and curse. Bipolar disorder, a syndrome in which the patient cycles from being extremely manic to extremely depressed, can also cause uncontrollable cursing. People who have suffered from a stroke (which is like a heart attack in the brain) or other forms of bleeding in the brain can also lose control and curse. Other chemical imbalances and brain damage can cause uncontrollable cursing fits.
There are millions of people every year who suffer from these conditions and cannot control how they speak. A completely healthy person who has total control over everything they say and still curses is almost mocking the people who cannot.