The answer to 'why is my deck yellowing' is that the wood has likely been exposed to UV radiation from the sun, causing it to gradually fade and turn yellow over time. This is a common issue with decks made from untreated or improperly sealed wood.
The yellowing is a sign that the wood's natural oils and protective sealants have broken down, leaving the wood vulnerable to the elements. This can happen more quickly in areas with high sun exposure or in humid climates. To prevent and address this, it's important to regularly clean, sand, and reseal the deck to maintain its appearance and protect the wood.