Amongst the twitterati, there seems to be a contest to get the most followers. No different from the Facebook, "let's see who can get the most friends" behavior.
The standard approach to increasing your follower count seems to be, find someone who isn't following you, follow them. They get the email saying that they are being followed by you. Being relatively polite, they may well follow you back. Once they have done that you drop them (in case their count is getting high and can threaten yours).
This is a form of bait and switch - I think I will name it bait and tweet.