People are saying steroids, but that's not entirely true, anabolic steroids don't cause this, but human growth hormone and possibly exogenous insulin use are most likely the culprit. Bodybuilders are well aware of this phenomenon and although we still aren't 100% sure the exact mechanism, this started happening to bodybuilders who started taking insulin and HGH (in addition to anabolics) in the late 90s and early 00s. Either one, or both of these drugs cause the organs in the torso to grow (among other things) which causes what bodybuilders coined, Palumboism, named after the first guy to show up on stage with a noticable gut despite good conditioning (low body fat).