I believe @iamthegray is correct—there are accounts that are set up to automatically follow new accounts. Whether they actually do any autovoting on your account may or may not be the case. As far as I know, none of the auto accounts that followed me when I first started, or who glommed on since are actually upvoting any of my posts. Depending on their intent, some of them may have wanted a follow for a follow, while others were probably hoping for upvotes on less than meaningful spam comments.

There certainly is a lot of bot accounts here, all with the intent of getting rewards in some fashion, without necessarily putting in much time or effort to do it. It's like mining, only proof-of-brain style. There's probably an oxymoron in there somewhere. :)