I too have seen my number of follows ratchet up substantially after a post, sometimes mere seconds after it's gone live. I think the logic behind all those auto-follows is that if you program your bot to follow a whole slew of people, you can monitor all those accounts, and when there's a major upswing in activity, tell your bot to vote and get it on some of that curation action.

Otherwise, I have no idea what would possess someone to want to be following hundreds or even thousands of people. If they post regularly, there's no way you can possibly read, enjoy, or curate all of that content.

I don't think there's any real reason, it's just people trying to game the system. I may have over 600 followers, but it's basically the same dozen or so people who vote and comment on my content. I'm lucky enough to get @curie votes from time to time, and I think that pulls on more robo-followers. I also know a number of people who follow me have stopped using Steemit, or at least left the platform for a while. There's no way to 'drop' those followers, so they linger in a sort of limbo: both here and not here at the same time.

We grow as a community by introducing new people and continuing to put forth a positive effort. One real follower is worth one hundred bots, after all. Create content people want to read and resteem, and we reap the rewards. :)