The reason is a fungal infection which affects the buds and flowers of the rhododendron. The initiator is the fungus pycnostysanus azaleae. But it only gets access through the rhododendron cicada which punctures the plant to subsist itself.
Fight the cicada already as larvae during hatching – which is end of April till mid June. Spray a environmentally friendly pesticide, for example, based on natural pyrethrum, two or three times in a weekly interval. Be aware that the larvae are mainly on the bottom of the leaves. The first thing, at any rate, is to take away the ill buds and flowers to prevent the spread of the spores.