There’s two reasons to do this - as an interviewer, the way you present yourself to me is the way I expect you’ll present yourself as a representative of the team or company. It’s not good to air dirty laundry publicly, and if you can’t keep it under control when it’s almost an explicit “don’t trash talk your old job” the. There’s basically no hope of it when things are more relaxed.

Secondly, there’s three sides to every story. Yours, theirs and the truth. If I say “my manager is an asshole so I’m leaving his team”, it’s probable that my manager has a different take on it, maybe “maccard said they wanted to be kept in the loop so I am telling them what happening but they accuse me of changing my mind”.

Neither of those answers seem at all dishonest. They just skip past the diagnosed cause to focus on the impact.

Maybe the boss being a raging ass is the root cause of the team moving in the wrong direction. But that may be debatable, and knowing that doesn't answer the interviewer's question. The more relevant piece is that you and the director disagree and that you don't want to waste your time working on something you aren't inspired by.