Some people in fandom who are triggered by such things as noncon, character death, etc. want authors to warn for them so that such people can avoid them. Other people want authors to warn for such things because they simply do not want to read them. Still others want warnings because warnings will help them seek out certain fics.
However, other people do not want to warn, nor do they want warnings made mandatory in any way, because they regard warnings as spoilers. During wanks between the two sides, solutions such as spoiler warnings, formatting spoilertext in white so that it must be highlighted in order to be read, and "warning not to warn" are generally disregarded.
This wank first received wide fandom attention when metafandom and unfunnybusiness covered the June 2009 wankery that started with warnings about a bandombigbang fic. The story then went wide and triggers became a frequent topic of discussion.