Causes: When men run, their nipples are constantly rubbing against their shirt. Over the course of a run (especially a long one), this sensitive area can be rubbed to the point of bleeding. Because women wear tight-fitting sports bras, this shouldn't be an issue for them.
Prevention: Some men learn the hard way how painful it can be, but it's actually very easy to avoid that problem. Generously apply a lubricant like Vaseline or BodyGlide to the nipple area before a long run and you should be fine. Some men will also wear products such as Nip Guards or Band-aids to protect the nipples. Also, for longer runs, make sure that you wear a synthetic-material (Dri-Fit, not cotton) shirt closest to your body. Cotton shirts will cause chafing. Women should make sure their sports bras are not cotton. For longer runs, both men and women should also apply Body Glide or Vaseline to any areas where there may be rubbing (inner thighs, under arms) to avoid chafing.