Hi Avedon,
Well, of course the plants could have just gotten a little too much water. However, it might be a good idea to check the plants for spider mites, just to make sure. I understand that butterfly bushes are commonly afflicted with them. They start showing up around this time of year too.
Hold a piece of light colored paper under a branch and tap the leaves a few times. Then check the paper for small orange moving dots. Spider mites are very small, about the size of a period at the end of a sentence :-)
Art
