Epidemiologists call this a "cluster," a burst of cases occurring together in time and place.

More specifically, do we see clusters of new languages occurring together over time interspersed with periods of stasis?

They track crimes, look for clusters where many crimes are occurring together, then flood those locations with police officers.

It is the idea of two or more events that are apparently unrelated occurring together in a meaningful manner.

Conclusions: Panic and rapid mood switching occurring together in familial bipolar disorder may define a useful subphenotype for future studies.