Wikipedia's entry for Conspiracy theory is illustrative:
Conspiracism
When conspiracy theories combine erroneous 'facts', observational fallacies and lack of evidence, critics refer to them as a form of Conspiracism, a worldview that sees major historic events and trends as primarily the result of secret conspiracies.[1]
According to many psychologists, a person who believes in one conspiracy theory is often a believer in other conspiracy theories as well. Belief in a conspiracy, or even conspiracies, is not necessarily a sign of psychological problems.Conspiracy: A flawed world view
Some people distinguish between accusations of conspiracy and unfalsifiable conspiracy theories and argue that when conspiracy theories are proposed, the proponents bear the burden of proof. In justifying the classification of a conspiracy theory as conspiracism, detractors tend to level accusations that the theory is:
1. Not backed up by sufficient evidence.
2. Phrased in such a way as to be unfalsifiable.
3. Improbably complex or lengthy. A rule of thumb called Occam's Razor is often cited. It states that the simpler a theory is, the more likely it is to be right.
Defenders point out in response that:
1. Those powerful people involved in the conspiracy hide, destroy, or obfuscate evidence.
2. Skeptics / apologists are not (in their opinion) prepared to keep an open mind.
3. Skeptics / apologists may be politically motivated and have a vested interest in the status quo as a shill or agent..
4. Skeptics / apologists may be victims of a human tendency to assume the safest and most secure of all possibilities.
Epistemic bias
It is also possible that certain basic human epistemic biases are projected onto the material under scrutiny. According to one study humans apply a 'rule of thumb' by which we expect a significant event to have a significant cause.[2] The study offered subjects four versions of events, in which a foreign president was (a) successfully assassinated, (b) unsuccessfully wounded, (c) wounded but died of a heart attack at a later date, and (d) was unharmed. Subjects were significantly more likely to suspect conspiracy in the case of the 'major events'—in which the president died—than in the other cases, despite all other facts available to them being equal.
Another epistemic 'rule of thumb' that can be misapplied to a mystery involving other humans is cui bono? (who stands to gain?). This sensitivity to the hidden motives of other people might be either an evolved or an encultured feature of human consciousness, but either way it appears to be universal. If the inquirer lacks access to the relevant facts of the case, or if there are structural interests rather than personal motives involved, this method of inquiry will tend to produce a falsely conspiratorial account of an impersonal event. The direct corollary of this epistemic bias in pre-scientific cultures is the tendency to imagine the world in terms of animism. Inanimate objects or substances of significance to humans are fetishised and supposed to harbor benign or malignant spirits.
-Wikipedia
9/11 conspiracy theories
The freemasons