I think it's the threshold of the heat that they can handle. From what I have seen with my succulents, since all of them are in containers, once the temps start hitting high 90's to a 100F, they would really like to get some watering..beyond the 100's the plants just slow down to grow, conserving their resources..I see them dropping leaves, closing up their rosettes, stems buckling. And I guess that leads to the rotting..if we tend to suddenly overwater at that point. My plants get shade in the afternoon, but even then ambient temps are at just hot..settles at the very high 90's to 100 on very hot days..especially during the heat wave.
What I usually do is slowly acclimate the plants to accept watering a couple of days ahead, by stepping up watering, doing it early in the day, so by the time the forecast says 100's++, the roots are still ready to take in water. If not, they seem to drown and rot. I also try to water just at the root zone, not the leaves.
