The Gullible Mind Explained by Mike Adams.
Mike has captured several aspects of gullibility in this piece. The lack of critical thinking, undue deference to authority, not suspecting hidden agendas etc. This is a good article. It does of course emphasise the angle that its the failing of the individual that creates gullibility. I still feel that situation plays a major part. That said you are always cruising for a bruising if you have misplaced confidence in your own infallibility.
Also like any social phenomenon Gullibilty might be better explained by Structuration Theory which accounts for both agency and structure.