An "illusion or misperception involving a vague or obscure stimulus being perceived as something clear and distinct... explains many UFO sightings, as well as the hearing of sinister messages on records played backwards."

So... I would surmise that one experiences pareidolia when the brain's fuzzy logic matching algorithms get kicked into overdrive, so every random pattern begins to seem as if it echoes a person's own thoughts. In that case, maybe pareidolia is a manifestation of people's subconscious desires? Or their fears?