This is a common strategy to build followers, most likely with the intention to use the account later to send affiliate messages or something similar or to sell the account to somebody else, who just wants to boast a large number of followers.
Due to the limits of the number of users you can follow without a similar number of users following you, the strategy goes something like this: follow random users until you almost reach the follow limit (which is 2000 users, afaik), then wait a few days for users to follow you back. Unfollow some of the users that haven’t followed back and follow new random users. This should actually work until you have enough followers to go over the 2000 people limit and then gradually increase the number of followers/following.
Especially if an account is not tweeting anything or has just tweeted a few times in the past, this is a very suspicious practice, however it is being used often it seems.
Of course, the whole strategy is seriously flawed due to the fact that this creates a huge part of accounts that are essentially useless but have a lot of followers (which are useless again) since users employing this strategy will likely follow each other, count it as success for both parts, but will never read any of the “informational” messages the other users posts. (assume that you have a user like Alyssa_Milano or mashable who are followed by 1–2 million users and follow about 1000–2000, this may actually be interesting, since they cannot have achieved the followers by a building strategy and may even read the 1000 or so users’ messages they are following, while some self proclaimed “social media tutor” who has maybe 50000 followers and is following the same number is probably completely uninteresting)
(I should add that there are different types of twitter users around, so take this description with a pinch of salt)