The conventional interpretation of this phenomenon is based on cognitive dissonance. When people have taken "irrevocable" actions in the service of a belief—given away all their property in anticipation of the saucers landing—they cannot possibly admit they were mistaken. The challenge to their belief presents an immense cognitive dissonance; they must find reinforcing thoughts to counter the shock, and so become more fanatical. In this interpretation, the increased group fanaticism is the result of increased individual fanaticism.
In other words when any social movement, be it political, environmental, or spiritual, reveals itself to be untrue or inaccurate, only the most radical and unhinged remain with it.
It's interesting to note that mainstream Christianity has never become a bastion of the angry and irrational, perhaps because we cannot be disappointed when our belief that is neither locked in the past nor dependent on a nebulous future. We live for the now.