Did you hack Chartbuddy, slowly deleting his posts, from oldest to most recent? 
Now you're probably asking this, just to try to move yourself out of scope as a suspect? 
A clever move...

/irony off

No idea why, but it happens from time to time.
Isn't there a web page with WO statistics somewhere? You could track the posts-per-username figures (maybe programmatically using wget and grep as a start) to find out...