The answer to 'mulch turning brown' is that this is a common issue that can be caused by a few different factors. The most likely cause is that the mulch is simply breaking down and decomposing over time, which is a natural process. As the mulch decomposes, it can take on a darker, more brown color.
Another potential cause is that the mulch may be getting too much moisture, either from heavy rain or from overwatering. This can lead to the mulch breaking down more quickly and turning brown. Lastly, if the mulch was not properly spread or was applied too thickly, this can also contribute to it turning brown prematurely.