Could it be blackspot disease? This disease causes the leaves to yellow and fall off the plant. Unless you spray a good fungicide, to keep the leaves on the plant. I spray every two weeks and my leaves stay nice and green. Of course if you only have one bush, dig it up and replace it with a variety with less disease trouble.

I agree with dan, that blackspot fungus would be a major suspect. Spraying with fungicide according to label instructions could help fight this. Also, some varieties are less prone to infection.

Other possible causes could be insufficient deep watering during the hotter weather or spider mite infestation on leaf undersides, which causes them to turn yellow and drop off.

In hot dry sun spider mites can cause leaves to yellow and fall off quickly. They start at the bottom and work their way up. Deep water your bushes to help them be stronger. Shallow watering causes the roots to grow close to the surface. during a hot dry spell with little or no water the leaves will yellow and fall off. Spring rains help them get off to a great start. By June things get dryer with less rain so without sufficent additional water your roses can experience the symptoms you describe.
You can control spider mites by using a water wand to spray underneath the leaves. This washes them off. You need to repeat this every 2-3 days to break their breeding cycle.