It’s a new-ish phrase coined by some quack from Belgium and spread by Robert Malone, the schmuck who claims to have invented mRNA vaccines. It’s a fancy way of saying ‘mass hysteria’ and is a term used by dum-dums to try and convince other dum-dums that they’re somehow brighter than a 40-watt bulb.
https://www.forbes.com/sites/brucelee/2022/01/02/what-is-mass-formation-psychosis-robert-malone-makes-covid-19-vaccine-claims-on-joe-rogan-show/