I suspect that some of these folks are introverts working with a whole passel of extroverts who took to the social nature of Agile like ducks to water. Introverts need time and space to process stuff internally. If they don’t get enough time to themselves during the workday, they burn out.The Introvert/Extrovert point is interesting, and an area that I've explored over the years as well. I would like to say, though, that it isn't as simple as just introversion vs. extroversion.
I'm a card-carrying extrovert, and my full MBTI is ENFP. The interesting thing is that my combination of type indicators leads me to indeed be an extrovert, but I also need time on my own to integrate information. I'm energized by being in (and especially working with) groups while I'm there, but I do need to recharge later. I may seem impulsive, but that's generally because I've been thinking about a decision for a while already.
What I have found interesting in my decade in the 'agile world' is the over-representation of Intuitors (the 'N' in MBTI) that I've met. I did a poll on the XP list a number of years back, simply asking people their Myers-Briggs type. Only 7% of the respondents were Sensors ('S'), while 66-74% of the general population are S's. At one Agile 2010 session, I asked the question again and of around 10 attendees who knew their MBTI all were Intuitors.
I'm a computer geek and not a psychologist or sociologist (although as a coach I sometimes wonder...). I'm not qualified to draw any conclusions from very unscientific data such as this, but I do believe that where there's smoke there's fire.
When I first read about Extreme Programming in 2000 I distinctly remember thinking, "Yeah! That just feels right!" I didn't know it at the time, but that's the 'N' and 'P' in my ENFP doing their thing. I didn't need a stack of case studies or empirical data to tell me that Pair Programming was good - I just tried it. TDD sounded kind of weird, but when I and a colleague tried it, we were hooked almost immediately. We didn't need someone else to prove it for us, our gut feel said that the practices made sense and our own experimentation proved that.
I suspect, and have for a long time, that given the large majority of people who don't work by gut feel and intuition it's inevitable that there will be a backlash against practices that seem counterintuitive.
As Agile in whatever form you like moves into the early and late majority of Moore's adoption curve, it will encounter more and more of the majority of people who are Sensors. With that will come people who question the very foundation of some things that many of us have worked with for years. That will itself create friction, but it also represents an opportunity to further improve what we do now. If we can quantify why these practices are so good, we can likely also determine how to make them better.