This reminded me of thinking previously about this score - it seems Bernstein created a theme that was energetic and dynamic, almost carefree, in the midst of a horrible situation. It was almost sassy and jaunty as if to say - these guys know they will win. Steve Mcqueen was so cool in that... the perfect actor for it. He couldn't care less - he is ON it.
Anyway, Bernstein must have been thinking about something along those lines when deciding "how will I score this?..." A master of film music.