Mass occurrence—aggregation, blooming, or swarming—is a remarkable feature of a subset of usually diverse scyphozoan clades, suggesting it is evolutionarily beneficial. If so, it should be associated with one or more phenotypic characteristics that are advantageous and which facilitate occurrence en masse. Here, we examine the evolution of morphological, ecological, and life history characteristics of medusozoans, focusing on the taxa that occur en masse. By tracing the evolution of aggregating, blooming, and swarming phenotypes, organismal traits, and environmental settings on an up-to-date synoptic phylogeny of classes and orders of Medusozoa, we are able to hypothesize circumstances that enable taxa to occur en masse. These include character states and character complexes related to podocyst formation, strobilation, oral arms, large size, and shallow-water habitat. These evolutionarily advantageous traits may be adaptations that evolved in response to selection for individual traits such as survival during periods of few resources, feeding on pulsed resources, and fecundity. These adaptations were apparently subsequently coopted by selection for reproductive success which favored mass occurrence. By considering the distribution of traits describing other phylogenetic lineages—when appropriately detailed ecological and systematic descriptions become available—it may be possible to predict which species are evolutionarily predisposed to form problematic blooms if environmental conditions permit.