I can't speak for Reddit's admins. I can only speak as a moderator.
When I first started my subreddit, we had one problematic user that was mass downvoting all the comments. This was an issue for a young subreddit because there weren't enough upvotes to bring otherwise good comments back up. Eventually, I was able to "fish out" the user by posting content he would react to.
For upvotes, mods will usually have a sense of what is and isn't going to be popular with the subreddit. If something that isn't "usually" popular suddenly hits the top of the subreddit, it would look suspicious. Even if a moderator doesn't take immediate action from a mere suspicion, trying similar content with a similar tactic on the same subreddit could result in future moderator action.
Hope that helps!