The term yaoi has recently been misappropriated in American slang to apply to many areas of slash fan-fiction as well as shonen-ai. This is more prone to happen when the fanbase for a non-Japanese show or series draws a large number of fans from the anime/manga fanbase. This is starting to become common in American Comicbook slash fan-fiction, especially if the artist favors a Japanese type style. For instance, Harry Potter slash fan-fiction is sometimes labeled as yaoi despite its clearly British origins.
The term was expanded in the 90s to include anime/manga fan-fiction with similar themes as the original doujinshi genre. If this trend continues, yaoi and its sister term, yuri may eventually displace the term slash.
The prevalence for the term yaoi and the changing of its American meaning can be attributed to many things. People may simply want a shorter/easier word to use than shounen-ai, yet still want a term that is exclusive in meaning. The traditional American yaoi fanbase may have started becoming defensive and wanted to redeem the meaning simply because reading something that loosely means smut maybe unbecoming. The trend itself may have sparked in Japan and spread over, though my Japanese falls far too short to try and find out this avenue. Unfortunately, a wide spread study of fan-fiction connoisseurs has so far not been done and its nature makes it hard to document and keep track of.
Yaoi is increasingly becoming a formal genre on American shores. There is not the doujinshi divide here, as there is in Japan, so many people use yaoi in reference to any girl's comic that focus on homosexual relationships. While this sub-genre is typically referred to as shounen-ai in Japan, yaoi has become the operative word, leading to some issues of content as the more graphic manga can appear alongside it's less graphic cousin -- sometimes leading to the shock of some children.