It shows an unprofessional and inappropriate interest in things unrelated to work, and, from my experience, is a good indicator that the person will either micromanage, has poor work life balance, or is not socially fit to be a manager.
I know that sounds super harsh, but you should seriously reconsider. You're going to scare people away. There's a power dynamic that you don't seem to be aware of, during the interview process. It's not your concern what they do with their free time, unless they present it to you, unprovoked.
This is from someone who has interviewed hundreds of people.
Besides all of that, it opens you up to litigation for discrimination, depending on the answer they give. I suggest talking with your HR department. Where I am now, questions about personal life are strictly prohibited, for that reason.
edit: please note the "unprovoked". If people want to share the projects they do in their personal time, they will include it in their resume as a line item. I've seen it in probably 30% of the candidates. It means "ask me about these things".
edit: better yet, just rephrase, so it's not a potentially uncomfortable probe into personal time: https://news.ycombinator.com/item?id=45543812