I suspect that high IQ people are in fact more susceptible to conspiracy thinking than most. The randomness of the universe is scary. Sometimes bad things happen to good people for no reason. Much easier emotionally for some to live in a world where things happen for a reason, even if that reason is due to malevolent intent. At least a malevolent conspiracy can be fought or exposed or understood.
IQ measures pattern recognition. If one is mentally ill, or otherwise intent on finding explanations where none exist, one might see patterns where none exist. If you have a high IQ, the more plausible and internally consistent your narratives will be, and since you're generally right about a lot, all the more reason to believe that your theory isn't a theory at all, but reality.