A character who pulls a flip-flop in terms of proclaimed sexuality or suddenly, for no apparent reason, feels strongly enough about it to become the Token Minority of their particular series for that particular sexuality. This can be because the author changed his mind about a character, is catering to Fan Wank, or is simply providing Fan Service. In any of these instances it is usually frowned upon. Not that it's entirely unrealistic, but it's very jarring. Sometimes employed purely as an answer to the assumption that all undeclared characters in fiction must, by default, be heterosexual.