Hello. The bumps on your leaves are called galls and they are caused by an insect mite. This one is specific to buttonbushes. There is nothing you need to do but you can prune off the highly affected parts of the plant. That would be for aesthetic reasons. Populations of insects fluctuate and you may see more in one year and less in the next. The health of your plant should not be affected by the presence of these galls.