For example when the words "ban" "unban" or "moderator" are mentioned, more than likely the user is trying to reach out to the Moderators of the channel.
In a similar approach, we can also track users of the last X amount of messages sent because the user may also be likely to be replying to a comment. If the content of the previous messages match what the user is typing, then they will likely tag that former person as well.

I see that a lot of ideas posted regarding chat are about improving this tagging experience, and what better way to improve it based on what the user has already written down. :)