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."