What this video covers
Furniture pushed flat against an exterior wall creates a pocket of still air that never warms up. The wall stays cold, the room stays warm, and the boundary between them collects condensation.
The clip uses a bed against an outside wall as the example because it is the most common version, but the same thing happens behind wardrobes, sofas and headboards. You do not see it because the furniture hides it until the smell or the staining reaches the front.
The takeaway
Pull large furniture a few inches off exterior walls so air can move behind it. It costs nothing and removes the cold, still, damp pocket that growth needs.
Read the full written guide: Winter Window Condensation and Spring Mold.