I agree that AVL trees make a bad question. But if you're in an interview, you should just answer the question as best you can. If I was asking a question and someone tried to avoid it by changing the topic, I wouldn't feel threatened, but I would indicate in my feedback that they didn't answer the question.
"Why do you want to do that" might be appropriate when actually working, but not in an interview.
"Why do you want to do that" might be appropriate when actually working, but not in an interview.