It's a little psychological "tic". I knew a woman that was passive-aggressive and would giggle a little when she'd say something insulting or running somebody down....the idea being she could say she was "just kidding" if somebody called her on it and said she was sounding like a jerk.
Article several weeks ago about how Mitt "compartmentalizes" his "personal life" from his "professional/political life". He thinks of himself as a "good person", a devout Mormon....in his dealings with families and friends (which he probably is).....but that it's "okay" to lie, cheat, and steal in his public life because that life is not "real" and thus it's not "sinful" for him to do whatever he wants to other people as long as he's not messing with his family and friends.
That would play into it too.