The forum system automatically changes your user title depending on your post count.
You might have spammed or someone thought offensive of your post(if it was deleted, meh.)
Choob is a set post count title.
Custom titles can be anything like I'MYAHORE or etc.
EDIT: Set post titles change your current title immediatly once you have reached the set post. If you have a custom title, the custom title will still be shown instead of the set post title.