I have several plants that go dormant mainly as they are done blooming. Those plants have no rebloom genetics or evergreen tendencies. Those plants are also hard dormant and are very winter hardy- although most of my other plants that remain lush are also very hardy as well. There are only a few that will begin going dormant even before first blooms appear. They will then get a few small leaves as the season nears an end before frost. They don't seem to be affected by heat or lack of rainfall, because they do it regardless of weather conditions. I have also noticed that many of them have narrow strappy foliage, rather than the wide foliage influenced by evergreen or reblooming genetics. In my garden, those with wider leaves can get sunburned easily in the early stages of rapid spring growth if it becomes really hot. Those plants don't seem to go dormant though and continue producing new leaves throughout the growing season until they are frozen back. Those going summer dormant will only produce one or two small new leaves no matter what kind of care they are given, and no more. I tend to notice it most when I am moving clumps. Those with lush foliage can be carried easily by the plant, but the dormant leaves will let go from the weight when you pick them up.
I can say that I do have a few newly acquired plants that look a bit like your photos, and those plants have remained small. We haven't had the heat related stress that many places are experiencing, and while it is dry, it isn't overly dry or droughty yet. I haven't ever had rot, so I can't comment about that aspect or if plants share any similarities. While it might be unsightly, part of me really doesn't mind that some go dormant. If I were overly ambitious, I would try harder to put those plants in their own section of the garden, but.....
Weeds seem to be doing exceptionally well though!!! Crabgrass is killing me!
