made popular by Al Pacino in Scarface, "mang" is the term for "man", but skewed into sounding like "mang" when said with the cuban accent of "Tony Montana"
Plain and simple, this was started by the Mexicans. It's their way of saying "Man".. This is due to the Mexican accent converted to English. Now people say 'mang' because it's (1) different and (2) humorous.