It's like plant tumors! have seen this with my older Rhodedendron bush -- I mean it's like 65 years or more old. it has one basket ball sized crazy round clump/ ball of crazy out of control clumping flowers on flowers -- the general appearance is a tumor/ abnormal growth. the rest of the old thing is sparsely covered with half hearted normal blooms when it does bloom. The crazy one reappears every year.

Read this:

Kumar R, Jamwal VVS and Sharma A (2015). Natural occurrence
of secondary capitula phenotype in Calendula officinalis L. Journal
of Plant Development Sciences, 7(12): 859-862.