Ben Yagoda has been on a quest:
I have for some time tracked the pluralification of sports-teams names. I am referring not to issues of what to do when the name itself is a singular or collective noun, such as Miami Heat or Utah Jazz, or to the British custom of using plural verbs for seemingly singular names. (“Manchester United are playing tomorrow.”) Rather, in the case of a team called the Cityname Nouns, historically (roughly pre-1980), it was customary to refer to “a Noun fan,” a “Noun game,” or a “Noun player.” This was analogous to other situations, where one would call someone who loved cookies “a cookie [not cookies] lover” or a place where shoes were sold “a shoe [not shoes] store.”
However, things started to change dramatically in the ’80s. Today, the norm is to talk of (for example) a Yankees game and a Yankees fan; the use of “Yankee” in those contexts is pretty much limited to the over-50 set. I’ve discussed this change, and the possible reasons for it, at greater length in my Chronicle essay “The Elements of Clunk” and in my book When You Catch an Adjective, Kill It: The Parts of Speech, for Better and/or Worse, but if you don’t care to read those texts and are at least moderately curious about the issue, you could take a look at the Google Ngram chart below, which graphs the use of the phrases Cubs fan and Cub fan in American English between 1950 and 2008 (the most recent year for which Google has data). Until the ’70s, Cubs fan was virtually nonexistent, but it overtook Cub fan in 1979 and by 2008 was more than three times more common. […]
I have been waiting for the day someone will talk about a ballplayer being “a Yankees.” Such a locution may seem absurd—but in 1930, talking of “a Yankees game” would have probably seemed absurd, too. And it made sense that either the Red Sox or the Chicago White Sox would have been the pioneer. “Sox” is obviously meant to be plural of “sock,” but the spelling obscures the etymology. And further, where it might make a certain sense for a player for other teams to be called a Giant, a Cub, or even a Cardinal, it’s kind of cuckoo to think of any human being as a “sock.” As a result, references to a Red Sox [as opposed to Sock] fan or a Red Sox game became common before such formulations were the norm for other teams.
He thought he’d found his Great White Whale when Kevin Youkilis, the new New York Yankee, was quoted as saying “I’ll always be a Red Sox.” Alas, it turns out he actually said “I’ll always be a Red Sock”: “The writer of the article I read had taken it upon himself to change the number of the s-word.” This is a change I had vaguely noticed but had not realized had become so universal so fast. As a traditionalist baseball fan, I deplore it (along with the designated hitter, interleague play, and teams changing leagues whenever they feel like it—I’m looking at you, Houston Astros), but as an observer of language, I note it with interest and only a minimum of teeth-gnashing. In any case, do look at his Google Ngram chart; it’s quite convincing.