According to Twitter’s own feed, it has disabled the number of followers displayed on user’s pages “temporarily … while we fix a bug.”
The bug is reportedly one that allows users to make anyone follow them, and Twitter users were apparently exploiting the bug.
UPDATE: Twitter says it has identified and fixed the problem: “We identified and resolved a bug that permitted a user to “force” other users to follow them. We’re now working to rollback all abuse of the bug that took place. Follower/following numbers are currently at 0; we’re aware and this too should shortly be resolved.”