Schizophrenia and pranksters both exist. Also people who want to get notoriety by saying anything they can to get attention.

Certainly much of the whole present "flat earth" popularity was driven by a bunch of people who thought it'd be funny.

I take just as seriously people who insist UFOs are aliens as those who insist the world is flat. I'm sure you can find somebody on youtube who has been "shown some documents" about how the world isn't round.