The answer to 'why is my fence yellowing' is that it is likely due to weathering and exposure to the elements over time. Fences made of wood, vinyl, or metal can start to discolor and fade as they are subjected to sunlight, rain, and other environmental factors.
Some common causes of a yellowing fence include UV radiation from the sun, moisture and humidity, and even mold or mildew growth. The discoloration is usually a sign that the fence is starting to deteriorate and may need some maintenance or repair.