Ever work in a machine shop? Any machine using cutting fluid can end up stinking like rancid butter in a very short time. There are additives that are supposed to kill off bacteria, but even that doesn't totally solve the problem.
At one facility I worked at there was a sudden shut down, and despite the machinist's advice to clean up and drain the machines, they were told to leave things and get out. After a month long shut down the entire place smelled like a cross of rancid butter and rotting fish...and that aroma would bond to everything...and hair in particular. It got better after a while, but never really vanished entirely.
There are some pretty strange anaerobic bacteria that can grow and multiply in some pretty bizarre environments. Anaerobic bacteria tend to give off methane as a waste product. (Aerobic bacteria tend to give off carbon dioxide. Facultative bacteria can live in either aerobic or anaerobic conditions and can give off either gas - tricky!)
I always figured that heat and fluid/mechanical action killed off bacteria, but a few always survived, and they would just feast on the dead ones and the population would explode again. Even the original oil isn't added to the forks in a "clean room". Always going to be a few little critters getting into the system somewhere.