There’s a potential link to folie au deux here.
Most of what we as human being believe we accept as the result of social proof, rather than other kinds of proof. In other words, large portions of our mental model of the world are socially constructed: we accept things that our peer group accepts, even if they contradict our experience. Strange and delusional beliefs can easily take hold in social groups that are isolated from the greater population or have no impulse to share the same ontologies as the outside world (say, cults, conspiracy theory groups, and insular societies with enough power to avoid having to kotow to public opinion). The smaller the group, the easier it is to diverge further and further from consensus reality.
The halo effect also feeds into this. A group often feels the need to believe the opposite of whatever its opposing group believes, regardless of the relationship these predicates have to reality; a group will handicap itself in order to show group solidarity and such handicaps often come in the form of a ritual show of spite to some imagined outgroup, sometimes as a clearly absurd belief. The absurdity of the belief is proportional to the strength of group membership. Believing that Obama is a secret muslim is the right-wing equivalent of the Yakuza cutting off their own fingers: it’s a sacrifice that is capable of only symbolic utility but very concrete pain, and so the symbol gains strength from the pain.