We can't determine from the photos what the exact cause of dieback is, but suspect it's a blight or crown rot of some sort. (Those are catch-all designations that involve a variety of fungi and sometimes bacterial pathogens.) One example of a commonplace summertime disease is the fungal infection called Southern Blight. You can find information on the linked page, including a photo example of the characteristic fungal capsules (sclerotia) to look for, though not finding them doesn't necessarily rule-out Southern Blight. Sometimes crown rots and stem rots are favored by crowded conditions that limit air circulation around plant stems and crowns (the main growing point, where roots and above-ground growth meet at the soil surface), especially during periods of wet or very humid weather. Often, pathogen spores require moisture to more easily infect leaves and stems. As a species of drier habitats, Butterfly Weed can also be vulnerable to root rot if the root zone stays too damp. (In comparison, the related Swamp Milkweed handles wet meadow habitats well.)

All you can do is to remove the dying growth, since it cannot recover. Whether the crown of the plant is able to regrow later is hard to tell, but if you can pull up the roots easily, then you will need to remove the plant since the roots system can't be rehabilitated. Otherwise, sometimes blights that only kill top growth leave the roots untouched, which can later regrow. No fungicide is helpful (it can't cure existing disease) or recommended in this situation, in part because such treatments may risk harm to pollinators or other organisms.

Miri