Oh, so sorry to see this, leungkitty. Here are some ramblings for you to consider as I look at the enlarged picture...

If this occurred in Spring to a mophead, I would suggest some kind of winter frost damage...

If the leaves had a tattered look, full of holes, look for insects but, even though that tattered look is missing, look for spider mites around the "new" foliage & flower buds...

If the soil was not sufficiently moist on a recently planted Bobo, the plant would abort the blooms due to heat stress (high temps, low moisture, too much sun, etc), transplant shock and-or lack of water. The plant may also show some leaf browning.

If a dry plant had been given fertilizer or aluminum sulfate recently, it may develop fertilizer burn on the shallow roots and this injury would make the plant abort the flower buds and some foliage (it is best to make sure the plant is well hydrated before applying these chemicals).

I could not see obvious signs of powdery mildew, botrytis blight or grey mold but, it was just only one picture. This time of the year, I would think that the last two are most likely.

Thanks! I think it was a combination of lack of water during new transplant (I moved them a few months ago), combined with heat stress AND I used soluble fertilizer on them all at the same time... oh well, better luck next year!