Dropping the '-ing' in noun adjuncts

Solution 1:

I don't think that there is any one thing happening here; there is no grammar conspiracy behind this.

Even between similar (swim[ming] team and swim[ming] cap) pairs, there is no pattern. (If you look closely swim team and swimming cap have become more popular recently than their counterparts.)

With some cases, it's obvious that removing the ing suffix would be bad (sometimes in a way I can't quantify) and thus there is no form without it (or it is exceedingly rare). Examples:

• Living room
• Vending Machine
  • "Vend" is very rare in English
• Shouting match
• Parking meter, parking space, parking lot
  • Park, as a noun, is used as in "Yellowstone National Park"
• Cleaning person
  • The change would change the meaning of the phrase

In any case, here is the data from Google NGrams for some pairs of words (click on an image to go to the Ngram page that generated it). You will see that the trends are all over the place:

Solution 2:

As others have already pointed out, both swim and swimming are nouns (or a gerund) and can thus function as noun adjunct. With regards to the swim team specifically first the proof that the question has some merit. The ngrams are conclusive for overall English, American English and even British English the term is on a quick rise. The last being the only one where swimming team retains a lead.

If we consider other teams, the ten most popular (ngram) all feature actual nouns rather than gerunds. Of course the four sports don't have an -ing ending. We don't go footballing, baseballing or basketballing and tracking is something else entirely.

In fact in the top one hundred noun teams there are only the engineering, marketing and the training team with an -ing ending. Although all of them have dictionary entries in their own right, rather than being conceived as a gerund. The search in the British corpus is similar. Thus, one could argue, that at least for the swim team it's just aligning itself to all the other teams out there.