The two main reasons I've seen it happen is because something unexpected happened, or didn't go the way the player wanted it it, or because the original concept of the character was unstable or incomplete.

My character is an unstable bitch, she broken in a sense and not without baggage. She does what she wants, when she wants and how she wants. She does have a conscience,  which is why she went to help out Tirarian. But she's not going to be nice to you, not right off the bat anyway, it isn't in her character. 

As a gm though, alignment helps me to anticipate the actions of a character.  Which helps for immersion. As I said, I haven't really seen it here, but I have seen it elsewhere,  and I'd rather be safe than sorry,  especially when this is something that I put so much effort into.