Something called proliferation - this kind of thing is usually caused by physical injury, such as a late frost when the bud is developing. Occasionally its a virus, but unless the whole bush starts doing it regularly, its unlikely to be that.

Thank you Bamboo the whole bush is not doing it, just ~10% of the flowers. So I'm to assume its the frost?

Sometimes high heat or excess nitrogen will also cause that.

Its any form of damage, so could be frost or even the hot weather in April. It'll pass.