My newly planted hydrangeas went from looking beautiful to dried up after yellowing. I tried to upload a picture but it’s not working. They are watered every other day in the early morning and get sun mid morning to mid afternoon.
It sounds like insufficient water. Whilst 20 minutes via your sprinkler system every other day might be fine for lawns or plants which have been present in the ground for a couple of years, it's not enough for newly planted shrubs, especially given your daytime temperatures currently. Hydrangea particularly does not like dry soil, so if you want to save them, they need to be given probably a full watering can (or a gallon or two) of water each every day, poured at the base of the plants on the soil so it has some chance of reaching the bottom of their rootballs as well as the upper parts of the roots. If they are in a really bad way, this should be done twice a day while the weather is hot and dry, or for a couple of weeks, then once a day for the rest of the summer.
If you dug those plants up now, you'd likely find their roots are still pot-shaped and bone dry; when your sprinkler is running for that short period of time, it will not be distributing enough water on the soil to soak those pot shaped root balls, which are probably 6-8 inches deep from top to bottom. 20 minutes with a sprinkler system will only wet the soil for the top inch or so, and the roots of any new plant take some weeks to 'break' out of the pot shape and venture out into the surrounding soil to gather their own water. Until they do that, they're dependent on you watering them deeply and sufficiently to keep them alive and healthy.