The answer to 'why is my landscape lighting yellowing' is that the lighting fixtures are likely experiencing UV degradation. Over time, the plastic or glass lenses on landscape lighting can become discolored and yellowed due to exposure to sunlight and UV radiation. This is a common issue that affects many types of outdoor lighting.
The yellowing is caused by the breakdown of the plastic or glass materials in the fixture. This can happen more quickly in areas with high UV exposure, such as in direct sunlight. As the lenses degrade, the light output and overall appearance of the fixtures will diminish.