When foliage on shrubs turns brown and drops off there could be several causes. This time of year (August) I always suspect lack of water first. If you do water and the leaves on only part of the shrub are falling off my next guess would be root damage from an underground rodent. Look for signs of moles or voles by feeling the soil around the shrub for runways or looking for entry holes. Then, a third cause for dropping leaves on part of a shrub could be insect or disease problems. Check the back of the leaves for brown pustules (rust disease) or tiny webs (mites) and clean up any fallen leaves to prevent reinfection. The good news is that your buddleah or butterfly bush is a tough plant and you can cut the entire shrub back (wait until spring) and then be rewarded with fresh growth and fresh start next year. Keep growing, Marianne Binetti