- Perhaps the writer simply doesn't understand what he's writing about.
- Perhaps he is applying his own Alternate Character Interpretation to them (of which, both Possession Sue, Ho Yay, Foe Yay, and Die for Our Ship contributes towards).
- Perhaps he's just writing a Transplanted Character Fic. Regardless, it's usually frowned upon to post stuff not labeled as such, with the term becoming quite negative if it gets applied to a story by other people.
- If the Fan Fic is of the 10%, this is likely because something in the story's backstory or plotline made them act this way.
Out of Character
A Sub-Trope of Canon Defilement. In Derivative Works (especially Fan Fiction), this term means that somebody is acting largely against his or her established personality. How and why this occurs has a pretty wide range: