Depending on where you are, and speaking only as one in the southeast, this is not so uncommon this year due to a combination of two things. One, the dry summer and fall last year made nearly all the blooms late this winter. And two, when spring arrives and the plant shifts into new growth it often stops pushing the buds. So some of the really late buds open poorly if at all and go bad as rains come. Not all varieties behave this way, and I don't know if that is what you have, but it sounds like something climatic because you say the plant is otherwise healthy. I wouldn't worry a bit, just give it a good pruning now and see what happens next year!