There's no single confirmed cause, but one leading theory is a brief timing mismatch in the brain: the areas responsible for recognizing familiarity may fire a split second out of sync with the areas processing the new experience, so the brain tags a brand-new moment as "familiar" even though no real memory of it exists. Another theory points to the brain quickly (and often incorrectly) matching a new scene's layout to a vaguely similar memory, creating a false sense of having lived through it already.