The growing trend in garment manufacture in the last few years is for brands to outsource manufacture.
A brand with a factory with spare capacity might manufacture for another brand, though usually the companies doing brand contacts are unknown and in the third world.
Jantzen used to be big, taking on work for Nike would keep them going until Nike subcontracted overseas.
Also manufacturing brands have been bought out by bigger multinational brands, eg Umbro in the UK was bought by Nike in the US and continue making football (soccer) kit.
In 1980, Jantzen was purchased by Blue Bell, and Blue Bell was acquired by Vanity Fair Corporation in 1986. This acquisition was a disaster for Jantzen and led to its demise. In 1995, the company dropped the production of menswear to concentrate on women's apparel, returning to its roots in swimsuits. In 2002, the Jantzen trademark was purchased by Perry Ellis International, Inc.[1] This included the licenses of Nike and Jag Swimwear.
http://en.wikipedia.org/wiki/Jantzen