I would suspect that the plants are thirsty, and would benefit from more frequent irrigation. Perhaps your soil is sandy, and water drains too quickly? Or the evenings are almost as hot as the days, aging the back petals prematurely?

Many master growers I know have automatic watering systems from 2-6 times each day, with just enough to moisten the soil without completely soaking the roots. Of course, that is for folks going for the biggest, most perfect blooms, and is truly not necessary for the majority of dahlia growers. Irrigation 3-4 times a week is enough for most growers.

In very hot regions (104+f/40+c), often successful dahlia growers use an intermittent misting system (every 15 minutes) and shading cloth to keep the blooms cool during the day. At your temps this summer, you shouldn't be needing to go to such extremes for nice blooms.