Hi Sheri, I do not grow butterfly bushes, but my plants exhibit similar dropping of leaves, even our city trees do it when we go into heat wave mode especially triple digits. It is the plant's reaction to the heat, they will drop the older, lower leaves and maintain only what they can sustain through the heat. If I see we have a forecast of days of high 90's to triple digits, I deep water a day ahead, so the plants can drink up and save up what it can then if heat wave continues, another deep watering mid week when it is still early in the day to help them drink some more before more baking heat ensues.
At least you got some humidity there, we have really dry dismal humidity, no rain since it is drought here, so lots and lots of falling leaves. The plants stabilize again when our temps return to the more tolerable high 70's to low 80's, I see them slowly making new leaf growths again.
