You were probably singled out because one of the anti-P2P companies (for example Media Sentry or Media Defender) had logged your IP address from whatever Torrent you where downloading and traced it back to your provider.My point was that I was surprised to be singled-out (which I presumed was because I used more bandwidth than I normally do)
In the past ISPs would bin the majority of the warnings they recieved from these companies, but after the Government issued its recent warning, ISPs maybe now feel more obliged to act on them.
Peerguardian is a good way to stop your PC from connecting to these companies. It's far from perfect, but judging from my recent PG log file, it does make a difference.
Not that I would condone the downloading of copyright material of course.