I never tossed my Reddit account when I left, so I still get notified of replies to my posts and comments; I’d say there’s a third type of bot - an “engagement bot” that takes high karma comments on old posts and replies to them in a manner that adds nothing but could trigger the original commenter to reply.

At first I thought it was actual people, but it’s always young accounts with high post volumes, all the same type of post that nobody who had actually read the original thread would have written. And the accounts seem to target high karma comments, and aren’t limited to any particular subreddit.