A recruiter will ask you this question for lots of reasons; there is no reason to get defensive. The main reason we ask is because hiring managers will ask us. So whatever you tell us we will relay to the hiring manager, with the goal of presenting you honestly in the best light.
Here are some tips on how to answer this question.
Tell the truth, even if it is bad. The recruiter will be able to find out what happened one way or the other, so just be up front. People are more likely to respect that you have been honest. If you were terminated explain why, and more importantly what you learned from the experience.
You need to stay positive in any interview setting, so do not use the time to complain about your previous boss. Speaking negatively about past employers will only make you look bad. You are not the victim in the situation, you need to speak professional about what you gained from your previous roles.
The best way to describe a bad encounter at a previous job is explaining that it was not a good fit. However, do not leave it at that, be detailed. Explain why it wasn’t a good fit in detail such as, I like a more team based environment, a smaller company with a less corporate feel, or my skills were not being used to their full potential.
If a recruiter has more questions about your previous job they will ask for references. This is where it is important to leave on the best terms possible, if you cannot give your previous boss as a referral that will cause you some problems. If you have a sticky situation when leaving a company try and be professional as possible, a good reference will help downsize any question a recruiter may have.
In closing, the most important thing for you focus on is the potential role. That is why you are talking with the recruiter, so point the conversation back to why you are interested in this job. For example, what interests you about the job, or what will be different? In my last role I wasn’t challenged in this area, or I didn’t get to work in teams.