The main threads that get closed are the ones in the 'introduce yourself' section, around the rest of the forum there is a number of reasons for the thread to be locked, such as duplicate post or a discussion has already been had about something.
In the introduce yourself section though, they get locked after a couple of days to stop people just posting 'Hello' messages and hence there post count rises without giving any true content or interaction with the rest of the community.
Hope that anwsers your question?
Dave