It doesn't need bots to explain it. And even the Reddit admins were convinced T_D had legitimate traffic, which is why they were reticent to kill it... they just didn't like the manipulation and brigading and so on. If they could show it was bots, they would've killed it much sooner.

https://old.reddit.com/r/TheoryOfReddit/comments/4fh8s9/this...

Normal humans tend to upvote things that are already upvoted. But normal humans don't tend to look at the incoming stream of posts.

Ordinary T_D users were refreshing the subreddit's front page. Mods stickied the posts they wanted to rocket to the top. This got them past the hurdle where very few people look at /new to give those all-important first few upvotes. Ordinary T_D users upvoted the stickied posts. The mods then unstickied them less than an hour later, because now they're "organically" at the top of T_D, and the upvotes continued to pile in, rocketing the post to /r/all

Meanwhile, all the other subreddits weren't playing this game, so their users votes were split across multiple posts on their subreddit's front page. And mods of other subs use sticked posts for administrative notices, which are worded as such and tend not to get upvoted much... but if you were to sticky a normal post, users would upvote it. But stickied posts aren't eligible for /r/all... unless you unsticky them. Oops! T_D successfully gamed that oversight.

EDIT1: Also... as the comments in the link above reminds me; it used to be that any post could be stickied, e.g. normal link posts. It wasn't necessarily clear that they were stickied posts. What changed after the T_D manipulation is that sticky posts were renamed "Administrative Notes" and had to be text posts and had to be coloured differently from normal posts. Before that change, they weren't distinguished that way. Now perhaps the subterfuge by mods makes more sense?

EDIT2: WIRED's postmortem on T_D - it was cunning mods, not bots.

https://www.wired.com/story/the-hate-fueled-rise-of-rthe-don...

T_D’s moderators were looking for a way to game the system and force T_D onto r/all every day.

The mods realized that a key lay in the “sticky” system, by which moderators could pin a post at the top of their subreddit indefinitely. The system was meant for announcements, rule changes, upcoming events, and other minutia of day-to-day Redditing. But any thread could be stickied, and stickied threads behaved the same way that any other Reddit thread did: They accrued points by vote, and more points boosted the thread closer to the top of the page. This didn’t typically matter, since a stickied thread was by definition artificially held at the top of its subreddit already. But the mods weren’t trying to make threads visible on The_Donald. They wanted to boost them onto r/all.

T_D’s moderators began to sticky threads unrelated to their rules or announcements. Instead, they promoted especially provocative user-created threads. This tactic quickly proved effective. Before long, T_D was elevating a post or two onto r/all day after day.

Another T_D mod, Alex, says the team kept in close touch not only with which threads were successful, but also how mods could encourage their users to vote on stickied threads and drive them higher in Reddit’s r/all rankings. “We trained our subscribers to upvote and comment in every thread,” Alex says. “That is how we originally gamed the algorithm.” Jessie, a third mod, says T_D’s mods made “repetitive requests” to the user base to vote and boost threads. They used memes, gifs, and jokes to push users to act. It worked.