I planted these double knock out rose trees about two weeks ago. At first, they looked great. I made sure to water, water, water – since it has been very hot lately. Now, they look sickly, the leaves are rapidly turning yellow and the roses (that were already in bloom) have all died.

What is happening to these poor guys and how do I reverse it?

It is Chlorosis - the leaves are producing less chlorophyll than they should be. This chlorosis, when leaf veins turn yellow but the tissue between the veins stays green, is caused by overwatering. That in turn causes a lack of oxygen in the root zone. And that causes the roots to die and the leaves to do what your rose leaves are doing.

You have watered them to death. Stop watering and hopefully, but no guarantees, they will survive. Keep the soil damp but not soaked.