https://en.wikipedia.org/wiki/List_of_mass_hysteria_cases
If I had to guess most of these went down because the child or relative of someone powerful, authoritative, and very disconnected from reality (gullible), did some bullshit the powerful person couldn't reason about. Literally, leaving their brain unable to compute reality instead choosing (and very likely believing) "bat shit" as the reason... Depending on how powerful the person who just skipped-a-beat of reality; the narrative might never be questioned. cough trump cough
I mean; like with UFOs, most of the imagery people use to describe are _known_ to them before the event. They're related to what the person knows and is capable of imagining.
With all that said, I'm not trying to explain them all as bullshit or nonsense; but, the list without the bullshit is a lot smaller. I do think it's a bit unfortunate though as I'd love to speculate and wonder about something besides the trite psuedo-hallucinations all these stories contain.