(This question implicitly includes, "Why do you want to leave where you are?" if you're currently employed.)

This question is an open opportunity to show you've done some research on the company where you're interviewing. All companies and interviewers are flattered when the interviewee knows who they are, knows what games they make, and wants to be a part of their experience. Do your homework and put on a good show!

Don't say things like, "I need a job," or "I need to move to Sacramento." Instead, pick a few things that are germane to the company in question. The more specific your reasons are tied to the company, the better. "I want to work on FPS shooters" isn't as good an answer as "I want to work on Game Franchise X because I played the first two games and still see potential for future growth of the product." It's sycophantic, yes, but interviewers are as prone to flattery as anyone else -- although don't give that as your only reason.

When explaining why you want to leave your current job, the trick is to not be negative. Pick a couple of points that are inarguable, for example, "There was no career development" or "They weren't working on the kinds of games I'm interested in," rather than "Their management is clueless and they are going to die soon." The game industry is a small community -- you could very well be talking smack about your interviewer's close buddy.

If you were let go or fired, it's better to say something like, "We decided to part ways," or "It was my time to leave," rather than go into too much detail, unless directly pressed. In that case, the interviewer probably already knows what went down and is just looking to see what you'll say. Answer the question quickly and without negativity, and move on. You want to leave a positive impression.