There was an article I read once (I can't remember where. May have been in a psych class I took during undergrad) about behavior in cults. It was an attempt to explain a counterintuitive observation that, when a cult is centered around predictions for the future (ie. The world ends in May!) And these predictions fail, the members cult actually becomes more devoted, not less. You would expect the cult to get weaker after failing, but the members only become more certain that they are right.
It compared the process to evaporative cooling in physics. That is, when you have a liquid, molecules in it are constantly evaporating. Since only the fastest ones have the energy to escape from the bulk of the liquid, that means that every time one particle escapes, the average speed of the remaining particles goes down. Which means the temperature goes down. The same principal was at work with cults.
Every time some evidence contradicted the predictions of the cult group, some people would leave. However, since those people were (basically by definition) the ones most weakly devoted to the cause, the average devotion in the group went up with each shock to the system. Eventually you end up with a core group of true believers who have to deny basically all of reality because all the other people wised up to the reality.
/r/Sanders4President has been a great demonstration of this phenomenon.