According to the news report, the black man was talking loudly on bus. So for the comfort of all passengers, the middle aged Korean man told the black man to shut up. The gangsta-wannabe then obviously flipped the *beep* out. What made the black man angry was the phrase "Ne gah yeogi anja" (You, sit over here) and the black guy understood ne gah as nigga...
What a jerk! Zero tolerance for accents - he seems to have immediately jumped to conclusions about what was being said and, consequently, made the situation far, far worse.