In 9th grade science we learned that the bacteria that convert nitrogen to and from different forms are super-abundant in the soil and almost everywhere else on the planet (it was part of our "nitrogen cycle" unit). I would imagine the poor bacteria gets tracked *everywhere* and certainly some of could be attached to you and introduced when you stick your hand in the tank. Or attached to just about anything, really. Think about how many items go into our aquarium that we have handled... filters, gravel, thermometers, fish food, decorations, etc.
I think it's more surprising how many types of bacteria we manage to keep out of our tanks (pathogens, etc)!