I noticed that when I'm seeding a torrent (using Transmission, the default client under Ubuntu), it regularly connects to other seeds, to which, obviously, I won't send any data.
People I know have reported that they see the same behaviour using other torrent clients.
These connections appear during a few seconds, and then vanish. Most of the time they are incoming connections, but I think I saw some outgoing ones once or twice.
Why are these seed-to-seed connections made? The only reason that makes sense to me is that they are used to maintain the DHT.