I've been in this position and got some good advice from a friend: "My (current|previous) position was no longer a good fit for me" is not a lie and is about a

I've been in this position and got some good advice from a friend: "My (current|previous) position was no longer a good fit for me" is not a lie and is about as tactful as you can be and I would leave it at that. When probed simply explain that the environment had changed and was no longer what it was when you started. It's okay to just say "it was time to move on". Anything probing beyond that I would be as professional as you can but tell the truth.