When a girl hasn't gone through puberty, her hips often haven't widened for giving birth. Such girls can easily die in labor, along with their babies. This is why girls who haven't completed puberty shouldn't be having sex. Unfortunately, Muslims are commanded to follow the example of Muhammad, who had sex with the prepubescent Aisha when she was only nine years old. The Qur'an also allows men to have sex with prepubescent girls (see Surah 65:4 and Muslim commentaries on the verse).
Notice that the reporter attributes the problem to "cultural" practices rather than to Muhammad.