Haven't read much Avengers fic myself, but this seems to be a pattern in most shipping fandoms. When people write shipping fic that includes other characters, they want to pair those other characters up. Maybe the writer doesn't like het and would rather pair up two guys regardless of canon. This becomes a trend, and maybe people like the way the characters interact (or look) together, so they start writing/drawing about that pairing specifically and it becomes a part of fandom.

As with everything, everyone has a different opinion on them. Then, of course, you get those people who insist everything is right and canon.