You don't explain the real reason. You never do unless it is "I am interested in pursuing more interesting opportunities after doing xyz here at my current firm

You don't explain the real reason. You never do unless it is "I am interested in pursuing more interesting opportunities after doing xyz here at my current firm and this job opportunity at your firm seems like a great fit". Yes, that should always be the reason. It is unfortunate but the reality of hiring world. Anything else you say can and will hurt you and could be the difference between you getting a job and getting a rejection email if at all. Never say "I left because of hostile env" because that will open up discussions about things that will not give you an upper hand. The new hiring manager can never truly believe what you said and will most likely think "this person complains too much..hmm should we hire this person". The reason is that this new hiring manager does not know you at a personal level yet. So even though you may think it is a good idea to share your story, they will only want to deal with simple reasons like "better opportunity", "closer to home", "love your product" etc.