Bud blast may occur for many unknown reasons but usually occurs when plants are subjected to cold weather after they had begun swelling.
Dead Heading
Breaking off nearly spent flowers is a good practice to keep flower diseases from becoming a problem. Simply snap off the old flower at the base of where the flower starts. (yes, it can be an overwhelming task).
Note that if you do not Dead-Head, the seeds pods will stay intact and you will potentially have to wait an additional year before the branch blooms again. Deadheading enables (helps ensure) your rhododendron to consistently bloom annually.