Dry rot occurs when wood is weakened due to a fungus infecting the wood. The fungus enters the wood through a crack or split and infiltrates the timber rapidly. As the cellulose breaks down and the fungus attacks, the wood becomes brittle and crumbles.
Rotting occurs when the wood gets wet, which makes it more appealing to the fungus. The fungi that cause dry rot need moisture, oxygen and wood to survive. As the dry rot enters the wood, it breaks it down and allows the moisture to permeate further, and the fungus spreads like cancer, which is why it's crucial to detect dry rot right away so you can remove the infected wood immediately.
If your couch is made of wood or has wood components, it's possible that it can dry rot during such a long time in storage. Consider purchasing a temperature-controlled storage unit and verify that it is properly ventilated to prevent moisture and condensation from creating an environment that invites dry rot.
Dry rot can be mistaken for damage from pests such as termites. It appears grayish in color and covers the wood in sheets. If it gets exposed to light, it may get a yellowish tint. Damage to wood can also be a combination of both a fungus and termites.