What caused the damage? Aphids.
These nasty little green insects in the picture above were clumping all over the spirea's stems devouring the leaves. Aphids enjoy sucking on the plant juices that come from fresh leaves and stems. As they eat they secrete a substance known as honeydew. It's such a nice sounding word for something that comes as a waste product from an insect. The honeydew then can attract a mold to further destroy your plant. Aphids can also introduce harmful diseases to your plants. You can see that even though they are small little creatures they are big pests.
What is crazy about aphids is their reproductive cycle. The little green girls in the above picture (and girls they are, nymphs) are hatched from eggs that are laid in the fall. In spring when they hatch the aphids become stem mothers and produce more females which will continue to reproduce more offspring. In the fall when the days are shorter and the temperatures are cooler the aphids will produce young with both females and males. The aphids mate and lay eggs then the cycle begins again.
What can you do about aphids? Insecticidal soap works well. I've used it many times on our lemon trees (in pots, they aren't hardy for Tennessee winters) and it usually takes care of them. A blast from a garden hose will knock them off your plants or you can rely on natural predators like lady bugs and lacewings who like to munch on aphids. You could go with a variety of pesticides but insecticidal soap will do the job fine and it is more ecologically friendly than most pesticides.
We didn't plant this little guy but we like him since he kind of regrew from nothing in the aftermath of the previous occupants of our home. Before we bought the house the front garden was stripped of everything that was there except for the nandina and apparently our little friend who sprung up out of nowhere. Although I suspect this little spirea is really a 'Magic Carpet' spirea (Spirea japonica) I would be tempted to rename it the Phoenix since it regrew from nothing. Now with our help this phoenix will rise again!
Be on the watch for these pests on new growth. If you suddenly notice leaves dropping or stem damage look a little closer and you may find what's bugging you!
Here is some good information on the little buggers.