It happens to physicists too, as satirized by SMBC [1]. Without knowing the first thing about psychology/sociology, I'd posit that it's because both have to learn what is socially accepted as a 'difficult' discipline. Since every other subject is perceived as similarly difficult or easier, their meager understanding of another field is magnified out to perfect knowledge (i.e. this subject must be easy to learn since it isn't physics/engineering && I've spent a small amount of effort learning it, therefore I am now an expert). It'd be interesting to see how experts in other 'difficult' fields react, for example, master martial artists or the like.

... but I'm not a psychologist, so take that with a heaping teaspoon of salt.

[1]: http://www.smbc-comics.com/index.php?db=comics&id=2556