Another reason
It seems to me that conspiracy theories tend to spring up whenever the official version of events does not add up, or sounds fishy. That leads people to not only suspect that something is being covered up, but to try to formulate an alternative version of events. That's when they often trip up.
For instance, in "the Roswell incident", the fact that the army changed its story made a conspiracy theory inevitable. In this case, it seems that there WAS a conspiracy - even if it had nothing to do with aliens and more with top-secret military experiments.
Of course, sometimes the conspiracy theorists get proven right. Then it turns into a "political scandal" and people forget that it started off as a conspiracy theory.
