The flow of a (possible) attack is something like this:

1. User configures browser to automatically start torrent downloads when a ".torrent" link is clicked

2. User clicks twitt button which leads to a torrent file

3. The file is downloaded and opened in a torrent client

At this point, one could imagine a specifically crafted torrent file which exploits some vulnerability of the torrent client to gain (say) arbitrary code execution and now the user is, to use a mild term, screwed.

This attack could be used by any malicious site, really, but it's easier to get people to click a twitt button rather than some link on some site and besides, by preforming the attack this way the attacker would infect a sizable chunk of all internet sites (any site that uses the twitt button).