The answer to 'why is my mulch yellowing' is that it is a common occurrence due to the natural breakdown of organic mulch. As the mulch decomposes, it can turn a yellowish or tan color. This is a normal part of the mulch's life cycle and not a sign of a problem.
The yellowing is often caused by fungi breaking down the organic matter in the mulch. This is a natural process that helps nourish the soil. However, if the mulch becomes too thin or sparse, it may need to be replenished to maintain the benefits for your plants and soil.