A pig manure pit is not something nice to think about, but imagine when a jelly-like greyish goo grows on top of it, reaching up to 4 feet in thickness, and it traps methane gas and if any spark happens, the entire place blows up, pigs and all. Then you MUST think about it.
Scientists are scratching their heads, doing lab tests etc.: so far no explanation. Some lay-people think it's a consequence of the antibiotics and GMOs that are fed the hogs. As the saying goes, garbage in, garbage out.