I am trying to build a world that appears to be much like our own but where the conspiracy theorists are right more often than they are wrong. Aliens did crash at Roswell, the Illuminati and the elves have been fighting a secret war, and the shooter on the grassy knoll was Elvis. It is easy to find motivation for most of the characters to keep the secrets, but I am having some problems with group motivations particularly I find the common justification that people would panic if they knew the truth unsatisfying. What would be some other valid reasons for the Air Force to keep the Stargate secret or the men in black to hide the aliens?
I am assuming hierarchical conspiracies where the army is keeping secrets for one reason, but the civilian oversight has different reasons and are probably controlled by people involved in other conspiracies keeping secrets for yet another reason so that there are often differences between the real reason for the secret and the justification for it given to the rank and file members of the conspiracy. I am more concerned with the justifications given then the real reason as the real reason is a plot element.
Edit: This question is starting to get some fun answers for character motivation (which I tried not to ask about as I thought it was off topic). I am looking for public or semi public justification. For example it is entirely likely the middle manager Bob will order someone to spy on his ex wife for purely personal reasons, but as he wants/needs to present an Image of a Good Guy he will put a reason on the paperwork that the new boyfriend is the brother of a silvot sympathizer.