When being asked, what's the most harmless way to address my past record of leaving Ph.D. program in a different field? Background

As the title suggest, a year before I'm in the current program I'm in, I left another Ph.D. program in a different field and got a master.

Issues

As for now, I prefer not to talk about it if I don't need to. Most people don't ask and don't care. However sometime a conversation goes certain way where inevitably I have to mention something in the past. For example (X indicates the other person in the conversation),


  X: So do you have a master?
  
  fmlin: Um, yes, but in a different field.
  
  X: What was it? Why didn't you continue Ph.D. in that?


or


  X: Where did you go for undergraduate? Did you go to graduate school immediately after you finished undergraduate?
  
  fmlin: [A short answer indicates I was once in another Ph.D. program but didn't finished]
  
  X: [Many questions out of curiosity]


The reasons I don't like to discuss my past are

(a). It's a record of dropping out. It sounds bad despite I think I'm not morally wrong.

(b). I believe I was abused by my former advisor and was not treated properly (see my other answers on this site if you want to know some detail). I had confrontations with him, and some other faculties in the same department.

(c). It takes long time to explain (b). and incomplete information on (b). sometime causes people evaluate me negatively.

(d). Discussing these things in the past can never help what I'm doing now.

Questions

So far I only had similar conversations with some fellow students and it's not a big deal. However, sooner or later I'll start meeting potential advisors in my current department. I worry that, if they ask similar questions, these things in the past will make me look bad.

As the title suggest, my question is, if I have to, what's the most harmless way to address my past record of leaving Ph.D. program in a different field? The goal is to not make people suspect I'm a problematic student (and I'm indeed not!) that confronts supervisors for no reason.
Based on the information you provided in the question, I think you should focus on reason (b). You should probably avoid using words like "abuse" or "confrontation", at least in the beginning of the conversation, as they come off as really strong and thus tend to motivate people to question deeper. State that you have had problems with the previous environment (including the adviser and the department) and that it was in the mutual best interest that you continued your career somewhere else. Avoid blaming anyone. If the conversational partner seeks to question further, reveal details to what extent you deem best, but again it is good practice to avoid bad-mouthing your former environment. Stay as objective as possible.

Like you said, most people don't care that much. For those that do care, you can't really stop them from taking it negatively (much as you can't stop anyone from taking anything about you negatively), but you can stay true to yourself, by being honest and objective without bad-mouthing your previous adviser, department, etc.