6 Million Unfollows Later, Twitter Moves To Silence ManageTwitter
techcrunch.com
techcrunch.com
I would imagine Twitter would like to increase the quality of the social graph rather than purely the size.
Direct marketers follow as many people in bulk as they can without being flagged as spam, a small percentage automatically follow them back, everyone else gets bulk unfollowed, repeat. By doing this you can eventually accumulate an account with thousands of followers which you can then pump commercial messages to. The system works but it's a volume game - if you can't automate the process, you can't make enough money for your time. By breaking tools that enable mass unfollowing, Twitter makes the automation harder and therefore fights spam.
ManageTwitter almost certainly knows this.
BaitCrunch title regardless
Thinking about this makes Twitter sound right