The answer to 'why is my mulch brown patches' is that the mulch is decomposing and breaking down over time. This is a natural process and can cause discoloration and patchy appearance in the mulch. The main causes are moisture, temperature, and age of the mulch.
Mulch naturally decomposes as microorganisms break it down, especially in moist, warm conditions. This is normal and actually beneficial, as it helps the mulch enrich the soil. However, if the mulch is very old or the decomposition is uneven, it can lead to brown, patchy areas. To fix this, you'll need to refresh the mulch by removing the old, decomposed material and adding a new 2-4 inch layer of fresh mulch.