I'm not an expert. But here are some theories
1- visceral fat deposits around organs from high dose exogenous insulin use.
2- Intestinal growth as a compensatory response to the caloric/food intake bodybuilders commonly eat. A normal intestinal tract is not meant to handle 8K worth of calories day in and day out.
3- Undigested meat which builds up in the intestines causing a bloated or distended look. Humans often die with 5-7 pounds of undigested meat in their intestines, it takes a long time to build up but this is the average person following an average diet. Now consider the amount of animal meat eaten by bodybuilders year round and this could accelerate and/or amplify the process.
3- High development of abdominal muscles, today's pro's are heavier than ever. I've seen it claimed by exercise physiologists that it is physically impossible to have another 40-60 pounds of LBM on the average pro without an increase in waist circumference. If it were possible then you would have guys with upper bodies so big they literally wouldn't be able to walk because their core would be to small/weak to support them.
5- *Theory* Growth of intestines through IGF stimulation mediated by growth hormone usage. It has been speculated that "GH gut" occurs because of the high concentration of IGF-1 receptors in the intestines. So high circulating levels of IGF from HGH use would lead to intestinal growth. This is a theory which has not been supported by the current clinical studies. Reason being normal individuals posses defense mechanisms (as mentioned before) to high levels of IGF which are involved in the process of how the body regulates it's hormone balance.