The answer to 'why is my mulch yellowing' is that the mulch is likely breaking down and decomposing. As organic mulches like wood chips or shredded bark age, they can start to lose their vibrant color and turn a duller, more yellowish hue. This is a natural process as the mulch decomposes and becomes incorporated into the soil.
Some common causes of yellowing mulch include exposure to sunlight, moisture, and microbial activity in the soil. Over time, the organic materials in the mulch break down, which can cause the color to fade. This is not necessarily a problem, as the decomposing mulch will continue to provide nutrients and improve the soil structure.