When moving from one job to another : interview
Most of the people who attend an interview are actually working in a job which they are planning to leave if they get the position. When the interviewer knows about it, they will definitely ask you the reason when moving from one job to another. You need to be as objective as possible in your answer so as not to sound like you blame anyone for your decision (colleagues, boss, or office politics), you do not sound frivolous and you do not come across as a problem employee. If you do not match say that you have reached a level where you feel you are stagnating and need to move on.
Another question you are likely to be asked when moving from one job to another is about financial security. Ensure that while you sound sufficiently concerned, you do not come across as panicky and desperate otherwise you will loose your salary negotiating powers.
You will also be asked to give a feedback on your ex-boss and/or your company. It is important here not to badmouth under any circumstances. Rather you should give high and positive feedback both to your ex-organization and about your boss showing them as vehicles for your enhanced knowledge and experience.
When answering about the past, you should come across as a mature and confident person; not as a bitter and desperate to move on individual. This will project a positive image about yourself and your capacity to handle discomfort and conflict in professional and personal life.