Answer: While I don't have a specific diagnosis based on the information in your question, it is possible that the spring problem was caused by some sort of stress -- perhaps slightly different drainage pattern so they received more or less water than the others, perhaps something happened to them such as a dog urinated on them or a vole chewed their roots, or perhaps they were simply "tired".
Cone flowers (Echinacea pupurea) are generally very sturdy plants, but like many perennials they may begin to lose vigor after a few years. Plants can be divided to restore vigor, or they can be allowed to self seed and then rogued out as the seedlings grow on. In fact, some plants are simply more vigorous depending on their parentage and early care.
Over time you will find that even the most trouble free plants are occasionally troubled by something. For wilting problems I would first suspect drought, and in my experience Japanese beetles, grasshoppers and even mildew will also attack these plants once in a while. With some careful inspection you may be able to figure out the cause.
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