Attrebus is a moderator, so I doubt it.
I actually have no idea as to what exactly happened. Last time I checked we had nearly 194,000 members. Today just at 167,000. Odd.
Only an admin would be able to enlighten us with the "why and how". Hopefully there was some Karma involved in the democratic process...
As for the how:
The Administration of a forum has the ability to change account information. They can alter, in no particular order: Usernames, passwords, email addresses, signatures, avatars, and permissions (what the forum let's your account do). They can also delete an account, and may choose whether or not to retain the posts made by a deleted account.
edit 1: in some forums, the admins can even see your personal messages. For most forums a few changes to the forum software would be enough to enable such monitoring, and some forums might have this feature by default. (Example: Forums that have pm's function as hidden posts and threads directly within the main forum structure, think of it as your pm actually being a hidden thread on the community discussion that only the participants can see)