In a game theory context, this is kingmaking. The player's goal is to elect C, but once the player starts to lose, the voter influences the outcome of the game (in this case the election).
This is a common problem with three-player and beyond games. For example, if I were playing UNO and I was losing badly (I had maybe half the deck in my hand), I could start to always change the color to, say, red.
The problem arises when players' ability to influence the game does not vary directly with their distance from their goals. In some games, the closer you are to losing, the less ability to affect the eventual winner you have, like Monopoly (which has a whole host of other problems of its own, but we'll ignore that).
Another example would be in many kinds of online games, where you might realise your character doesn't have enough time left to reach some checkpoint, so you go try to knock other characters around and prevent them from reaching the checkpoint.