Once all body functions stop, the process of decomposition begins.  Our bodies are havens for bacteria - in the air we used to breath (lungs), food we used to eat (digestive systems), our skin that was exposed to all manner of microbes.  If left to open air airborn microbes go in for the feast and other life forms that live off decaying material have a holiday.   We are 98% water and 2% chemicals.  All kinds of external material attack from the outside, while we are attacked by internal organisms.  Gases build and, (not a pretty visual, but true) we bloat like a balloon and, well, you know what happens to balloons when overfilled.  Look up info on the Univ of Tennessee Body Farm.  They are world known for the research they do on decomposition.