The answer to 'why is my shrub brown patches' is that your shrub is likely suffering from a fungal disease, environmental stress, or pest infestation. Brown patches can indicate a variety of issues, from fungal diseases like anthracnose or leaf scorch, to drought stress, nutrient deficiencies, or insect damage.
If the brown patches are isolated to a few areas of the shrub, it may be a localized problem that can be treated. However, if the entire shrub is affected, it could be a more serious issue that requires professional diagnosis and treatment. In either case, it's important to identify the underlying cause to determine the best course of action.