I think Cthulhoo had the best answer so far, but let me throw in my two cents. The (unconscious) thought process behind the boos, as I guess it, is: "I am watching a bad performance, and thus I am becoming associated with it. An observer who saw the situation now would think I am the kind of person who likes this kind of performance, which would lower my status. To reaffirm my status against this threat by dissociating myself from the performers, I boo them."
The other day, someone did something I didn't expect. It was something many people have done before; something that I thought of as very normal, but that I in no way understood and had not predicted.
As I said, this had happened many time before, so I wrote it off as "me not understanding people" or "people are weird" for a second, like I usually do, before realizing that "bad at" really means "lacking basic knowledge", which I had never realized before.
And then I thought "I should ask someone who is different from me why people do that, and eventually someone will have an answer."
But many people will have many more questions like this. So, what have you observed people doing time and time again, but never understood? Or something that you only understood after a long time or asking someone about it?
And can Less Wrong tell us, not necessarily why (I for one can make up evolutionary psychology fairy tales all day if I want) but what conscious thought process occurs behind these events?