I'm not sure but I would suspect some people start off by "following" a lot of other hubbers and many of them will automatically sign up to follow you.
However I think it's best to write the best quality hubs you can and if someone reads them and likes what you say they will probably sign up to follow you.
Another way to get followers is to answer other hubbers questions or place genuine insightful comments on their hubs.
Many people out of curiosity will click on your profile to see what else you have to say. If the subject matter appeals to them they may sign up to follow you. (Like attracts Like) :-)
Try not to get too caught up in the number of followers or your hub score. Your goal as a writer should be to write the best hubs you can providing information, entertainment, or raising questions. Best of Luck!