Do you mean the flower stalks or the leaves?
If the flowers then suspect snails, they love to eat the stalks.
If the leaves then this is natural, the leaves die off as the flower begins.

mine do that all the time, but they then lift themselves before they flower, not sure if its a lack of moisture or a natural thing, or wind as our garden is like a wind tunnel at times. but agree with Owdb, check for slugs near the bottom, iv'e had barren seasons due to slugs razoring the stems. just a note the flower heads seem smaller this year?

I have had a few grow with a sort of 'U' bend in the stem when they have gone short of water and then regrown later.