According to TorrentFreak, the Dutch anti-piracy outfit BREIN has gone after a number of torrent sites including the large Bulgarian tracker RARBG which had over 100,000 users.
ArenaBG and many other large sites have been taken down as well. It appears that the police and BREIN are aiming to take down Bulgarian trackers, and working to identify the administrators of the site.
Once their identities were found out, each was asked to a meeting where they were told that their sites needed to be shut down. If they were not shutdown, the police would physically take the servers.
ArenaBG, P2PBG, MastersTB and AvatarBG are all either shutdown or moving on to new hosts.
Torrent-BG, WordBG, and Zarta.org are all shutting down as well.