Certified GKH Gardening Expert
There are a few things that can cause this. One is damage from herbicide drift. Another is spray or water getting onto the foliage during the heat of the day and the sun burning the leaf tissues. Sometimes an insecticide or fungicide spray that was mixed too heavy or rich will burn the foliage and turn it brown. Neighborhood dogs peeing on the foliage will also cause the curling, turning brown and falling off of foliage. It's also possible reflected sunlight from house or car windows and even some garden gazing balls can cause a burning of the foliage like you describe. A lack of enough water when the temps are high can also cause this. Check out the American Rose Society website, see if you can find a rose society or Consulting Rosarian in your area, they can come do a garden visit and see what might be happening too.