I believe that's about the time we switched to the new version of the forum software.
All the old posts are still there, but the database doesn't track poster information past that point.
I don't think that's the reason. The "My Content" tab actively stops displaying results past a year ago from the current date. For instance right now it says:
There have been 714 items by Bacterius (Search limited from 28-June 12)
And tomorrow it would say "search limited from 29-June 12", and so on.
That said you can try to dig up your old posts via google. Try "site:gamedev.net "started by [your username]"" or "site:gamedev.net "posted by [your username]"" etc.. (with the inner quotes to get exact hits - also add in some date keywords if you feel extra lucky)