As an experiment, a sort of Twitter honeypot you could say, I followed every single one of them on a fresh dummy account, using a simple Ruby script with the Twitter gem.
Less than a day later I already have more followers than the original 237 I added. I haven't posted a single tweet, so no one has any valid reason to follow me, except to try to gain more followers.
I suspect many of them will unfollow me shortly, after they realize I won't follow them back. I'm saving all the "new follower" emails I get from Twitter, and in another day or so I'll compare them to my remaining followers to see who's guilty.
The vast majority of the unsolicited followers are people peddling some sort of bullshit get-rich-quick MLM scheme.