I used to be a mod of a small subreddit and I was a huge redditor a while ago. It was kind of an open secret that pretty much everything on reddit was manipulated by bots. A huge redditor at the time (unidan) was banned for using bots to pump his posts and comments, and that made bot manipulation a lot less secret for a while.

It's an anonymous site and you don't even need an email address to sign up, so it's very easy to spin up many accounts.