Pretty much just moisture is the cause, not the house collapsing or anything like that.
The moisture may just be right above and at this beam or box in the form of a Mouse Infestation or by simply being the only place where insulation isn't. Where cold air slams into hot air is where condensation happens...insulation eliminates the collision so condensation doesn't happen in a gross amount.
The moisture's introduction could also be from a Roof or Plumbing leak or even an improperly installed exhaust fan from the kitchen and/or a bathroom(s). Their exhaust may not be going outdoors and is just dumping humid house-air into the attic.
On a much more rare level, it could be the combined efforts of air-leaking Electrical Outlet and Light-Switch boxes, poor fitting Attic Access Panel, shoddy or clogged Clothes Dryer vent pipe and even incorrect Furnace and Water Heater exhaust pipes.
Nothing's expensive and you can likely handle most everything yourself, the stores even have electrical box foam insert gaskets for both outlets and switches...usually in the Weather-Stripping section owned by Frost King.