I saw a documentary a long time ago, where the commentator was talking about the England cricket team, back in the early days when you would actually get a cap with the emblem, for every time you represented the team. The term moved to other sports and then the Aussies picked it up and then the rest of the world...except off course the Americans, who hardly talk about 'caps'.
My memory of the details sketchy though, but it shouldn't be too hard to get the facts.