It turns out, Twitter manages your follow/unfollow activity very closely. Twitter’s automation rules and best practices shows how it manages follow/unfollow usage:
Automation Rules and Best Practices
“Twitter users often review the profiles and recent tweets of their new followers. Being followed in a bulk, automated fashion can make this process bothersome and makes Twitter a less fun place to hang out.
The only auto-following behavior Twitter allows is auto-follow-back (following a user after they have followed you). Automated un-following is also not permitted. If you find yourself frequently needing to un-follow large numbers of users, you might consider reviewing your criteria for following them. Please review our Follow Rules and Best Practices for a detailed discussion of following recommendations.
Also note that in the general Twitter Rules that “get followers fast” applications and services are not allowed. Do not surrender your username and password to them.”