The answer to why your deck is yellowing is that it's likely due to sun exposure and weathering over time. Decks made of wood, especially untreated or lightly stained wood, can start to lose their original color and develop a yellowish or grayish appearance as the wood is broken down by UV rays, moisture, and other environmental factors.
This is a common issue with decks, and it's usually not a sign of a serious problem as long as the wood is still structurally sound. The yellowing is primarily a cosmetic issue, but it can make the deck look old and worn if left untreated.