Environmental stress. Likely heat and/or short of water.

Wind and cold damage is indicated when the edges of the leaves and tips become distorted and turn brown. This occurs frequently in plants that may be in a colder climate than recommended or in a location with severe exposure. Provide protection from winter winds and winter sun. Note, boron poisoning or fertilizer burn will create the same symptoms except uniformly over the plant rather than just on the sides of the plant exposed to the sun and wind.

Here is a link that might be useful: How to identify problems on azaleas and rhododendrons