This is a known phenomenon at science fiction conventions as well, typically known as "con crud". It's not uncommon for a large percentage of attendees to all get sick immediately after an event.
It turns out conventions of any type are excellent vectors for spreading contagion.
And it should be almost tautologically obvious, but this is extremely common for music festivals, which adds in the additional challenges of low sleep and immunocompromise caused by drug usage (drugs like mdma are thought to be immunosuppressive in recreational doses)
It turns out conventions of any type are excellent vectors for spreading contagion.