I had to watch that milk drinking thing at the end credits twice to see if I had missed some hidden joke or something. No, it just appeared to be a tacked on "idea" to mask the brevity of the actual ending, i.e. BSW's about turn. It was probably just Linehan, directing the episode as he was, just saying to them "Can you act crazy for about 40 seconds, like you've just won a prize or something, while you drink your milk", there probably wasn't even any rehearsal for that bit, it looked just so utterly amateurish and embarrassing. Can you imagine Seinfeld and/or Larry David would have entertained anything remotely resembling that to finish a season finale?