This could happen if you use a fertilizer with a high nitrogen content, but it's more likely a reaction to the drought or to overzealous pruning. Ideally, you shouldn't do any trimming at all. Those leaves are useful in keeping the rose healthy and happy.
My roses have produced far fewer blooms this year because it's been too hot and I can't water them enough to make up for the lack of moisture in winter and spring. My clematises also bloomed much less than in other years, but it was a case of overly severe pruning in their case.
