I only noticed this after accidentally clicking on the last page of AIBU (page 9155 on my browser!) All the AIBU threads before 2007 are dated 01/01/2000 and have no content.
In Chat, it's all threads before 04/11/2004.
In Pregnancy, it's threads before 03/05/2001.
I didn't check any other topics, just wanted to see if there was a cut-off date or the same number of broken threads on each topic. But nope 
Doesn't seem any logic to it - and it looks as though several years may be missing altogether.
Hope you've got someone who loves ferreting around in databases!