Bacterial growth in the hot water tank. Happens to everybody. Best course of action, leave the water heater off and run 20 or 30 gallons of cold water through it. Industrial strength (but just as temporary) cure, drain the water heater and entire fresh water system and sanitize with bleach as though new.
Usually, being on city water (i.e., chlorinated/disinfected water) forestalls this reaction, but it will always occur if you just wait long enough. Only reason you never have the issue at home is that the water never sits stagnant in the water heater for weeks.