Once a username has been removed or lost from the database, the existing posts that were associated with it have nothing in common that I can access, apart from the IP address used when the post was originally submitted.
I found 33 posts linked to your previous IP address and the most recent post from that IP address was on April 02, 2010.
If the problem had occurred within the last week or so, I might be able to manually trace back through error messages and login details etc. to see if anything obvious could be found, but I don't think I would be able to easily locate a problem that occurred sometime within the last 32 months, so I won't even waste any more time looking.
I can only assume that it may have resulted from either data corruption on the server during a software update, or possibly a glitch in the most recent forum software.
Alan