Heat stress can cause the yellowing leaves, as can overwatering. I know when it's hot and shrubs look wilted our first thought is to water, but wilted leaves isn't always because they need more water. Wilting is the plants defense against the hot weather where it tries to conserve moisture. Usually the wilting disappears overnight and the plant looks fine the next morning. By over-watering you are essentially suffocating the roots - wet soils exclude oxygen which can suffocate roots. To avoid this, water deeply once or twice a week. Deep watering will encourage deep root growth which is what you want. Cooler roots and moist soil will help your butterfly bush endure the summer weather. Adjust your watering schedule and the yellowing should stop. Best wishes with your butterfly bush.
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