Throughout history, there are many causes and triggers for mass hysteria but it remains a difficult thing to understand. Part of that is because of how common the circumstances behind such events are and yet, strangely, how rarely they happen.
Take a reasonably sized population that finds itself powerless, either from an oppressive government or a drastic change in circumstances that are beyond their control, and they will react badly. That's simple human nature. In the absence of proactive opportunities, hysteria is almost inevitable, which is why it's happened over the years in some fairly notable ways.
From meowing nuns to an imagined monkey-man, mass hysteria is a bizarre and little understood aspect of the human condition...
Writer of humorous novels; The Accidental Scoundrel, and Tripping the Night Fantastic.
Find them on Amazon here - https://www.amazon.co.uk/Accidental-Scoundrel-Rochdale-Manor/dp/1499628226/ref=sr_1_1?ie=UTF8&qid=1522068925&sr=8-1&keywords=the+accidental+scoundrel