I doubt it's " the open carry mentality". I'd wager it's more of the "I'm tired of people questioning how I go about exercising my legal 2nd ammmendment right" mentality.
I'm a firm believer in the biggest reason people have issue with people open carrying is because 1) so few people exercise their right to do so and, 2) the media has conditioned people to think that there are 2 kinds of people carrying firearms, LEOs and bad guys. If you aren't wearing a uniform and are carrying a gun, you must be one of the bad guys, because that is almost all they ever show.
If the millions of people who legally own a firearm all decided to open carry those firearms, and do it all day every day, pretty soon people would pay as much attention to it as they do to someone carrying a cell phone. For this reason I have mixed feelings on general open carry. In today's society with today's attitudes, you set yourself up as a potential target by the real bad guys and you set yourself up as a potential target of John Q who wants to confront you because he thinks you have some kind of personality disorder for walking around "flashing" your weapon. A mindset already born out in this very thread. But...these open carry folks could be setting the example. Tomorrow there's more, the day after that more again, till eventually you reach that tipping point where open carry is so prevelant it's just another thing.
In all honesty I doubt that will ever occur for the simple fact I don't believe there are enough people who give a hoot about it to engage in it, but I can't really fault those that do.