There have been an myriad of automatic subscriptions to 2000 or even 50.000 accounts which tried people to follow back, proviode Google juice or klicks to pages attached to those accounts.
Twitter now both used the BLOCK-commends given by users and predictive methods to weed out this spam.
Twitter was really threatened by this practice: On some days 90% of my new followers where just mass spammers and SEOs.
So: Thanks Twitter, this is a more relevant problem than even fail-whales... (And with the spammers at bay the fails whales will also be less frequent.)
The consequence of this was BTW that my follower numbers first went down from 1442 to 600something and now back to 1300something within a day ;)