It doesn't sound enigmatic. It sounds like anaerobic respiration:
https://en.wikipedia.org/wiki/Anaerobic_respiration
> ... In aerobic organisms undergoing respiration, electrons are shuttled to an electron transport chain, and the final electron acceptor is oxygen. Molecular oxygen is an excellent electron acceptor. Anaerobes instead use less-oxidizing substances such as nitrate (NO3), fumarate (C4H2O24), sulfate (SO24), or elemental sulfur (S). These terminal electron acceptors have smaller reduction potentials than O2. Less energy per oxidized molecule is released. Therefore, anaerobic respiration is less efficient than aerobic.
In short, heavy rainfall in the last year combined with decades of organic waste disposal in the landfills (think yard trimmings, discarded food, and other organic matter) have resulted in an enormous, uncontrolled underground anaerobic respiration problem in Southern California.