The answer to your question can be found in Chapter One, which is a short biography of Maniac Magee from birth until the time he became a legend.
His life started out normal, but when Jeffrey Lionel Magee (Maniac's real name) was three years old, his parents died and left him an orphan. He went to live with his aunt and uncle, who hated each other. Although they lived in the same house, his aunt and uncle didn't speak to each other and wouldn't be in the same room together, so Jeffrey had to "visit" each of them separately, eating dinner with one or the other each night.
After eight years of living like this, Jeffrey Magee lost it! He was performing in the spring concert at school, his aunt on one side of the auditorium and his uncle on the other, when he pointed at them and started screaming during a song. When the song ended, he kept on screaming at them to talk. "No one knew it then," says the narrator on page 7, "but it was the birth scream of a legend."