I would suggest that sudden flips are more indicative of repression than anything else.
Agreed, though I don't think it implies that the person has been repressing their sexuality all along. It could be that a person committed to a long-term relationship during a period where they were strongly attracted to one sex, and then over time that level of attraction diminished and they became more attracted to the other sex. But over that period they may remain faithful to the partner, in which case that change in preference wouldn't have much opportunity for overt expression unless there were a break-up.
As far as the idea that one's tendency to prefer one sex over the other can change, I don't find that to be remotely surprising. It's happened to me, and it's pretty uncontroversial among most my queer friends, many of whom are fellow flip-floppers.
I've noticed that the older generation isn't as comfortable with the idea, but they came of age in an era where there was immensely more social baggage associated with who you boink, and bisexuality wasn't really even accepted among the gay community. That perhaps created an environment where people who do have a more fluid relationship with their sexuality would have been under pressure to suppress that ambiguity above and beyond the hostility toward same-sex sex.