Have you dug any of them up yet? To see if there are any grubs, beetles, etc?
It does look like damage from heat and water. Kind of steam cooks them. I see what looks like some dry crispy leaves (from drought/heat), then see green leaves showing signs of color change due to a combination of heat and water.
The ones that are still healthy looking can be saved by cleaning them up by removing all damaged parts. Let them dry out before replanting (several days to a week). The soil I see looks very wet. Let the soil dry out, clean it up from the dead debris and aerate it, it might be compacted. Add chicken grit/pea gravel to the soil for better drainage.
Twit, what are your thoughts, it's better when we put our heads together.
