The build up of gas in the body and the intense state of rigor mortis did not give the gas an avenue of escape. The buildup became so intense an alternative exit was created, through the stomach and out his belly. The elf who found the man spent hours collecting all his parts before altering anyone. What could they do? Nothing and the man needed a complete body. It was due to the severity of the mangled flesh that no one noticed the body was not whole.