Be honest but not too honest. Don't trash your current employer or manager; interviewers will always err on the side of assuming that you are the problem (because it couldn't possibly be management). You can just say any combination of the below:

• You're looking for more responsibility.
• You've learned a lot and are looking for a place to apply that experience to a more interesting or challenging mission or industry.
• You're entering a new phase in your career and want to get a change of pace.
• You're ready for more leadership.
• You're looking for something you're more passionate about.
• While you love your coworkers, the atmosphere / culture at the current company isn't something you can continue to grow in.
• Never say you want more money :) most of these reasons are all polite code-words for that, anyway.