Sometimes a spammer posts links in an old thread or has a link in their signature and posts in an old thread.
Other times people who are new see the related threads and read one of them, not noticing that it's an old thread, and then post, thus resurrecting the thread.
Then people reading the boards see the thread and post, not noticing the original date.
Consensus seems to be that resurrecting old threads on curriculum boards is fine... especially if you're asking for more information or adding an update, but it's generally bad form to do on the general board...especially about issues that are long over.