It's silly, but the reason it usually doesn't work is because when you copy and paste, it doesn't parse the BBCode. Without shortcuts, to mention a person using BBCode would would have to type something like [mention=1111]username[/mention]. The @ just does the [mention] tags for you. But because it's copy and pasted, it doesn't actually parse the @ command, and just prints it as plain text. But because the text editor will notice it should work as a mention, it'll appear as one when posted, and the mention tags get wrapped after, but no alert gets sent. It's like it applies the correct BBcode retroactively, in a sense.