The answer to 'why is my landscape lighting yellowing' is that the light bulbs or lenses in your landscape lighting fixtures are likely deteriorating over time. This yellowing is a common issue that can be caused by a few different factors, including exposure to UV light, heat, and weathering.
Over time, the plastic or glass lenses on your landscape lights can become discolored and start to look yellow or amber. This is because the material breaks down and oxidizes when exposed to sunlight and outdoor elements. Additionally, the light bulbs themselves may also start to yellow as the internal components age. This can result in a dimmer, less vibrant light output from your landscape lighting system.