I think back in the day, users would request the admin at that time to delete their accounts, but in order for the posts not get destroyed along so as not to mess up threads. The username of the account would just be renamed as anonymous.
I also think mostly this was the case where accounts were deleted due to some reasons but the posts remained there it was visible as guest account to other users as they were kept in the same way so you see guest name on profile but in realistic approach you have to register account on the forum in order to make posts.