[Admin]

Well, I can only guess, but maybe it's part of a hack (if it exists) that adds a "Your inbox is full" popup to pages, like "You have a new message". Or maybe that's already a feature... god I can't remember.

It's possible to limit this hack to admins, yes.
Replace this:

  $replacewords=array(0 => "$bbuserinfo[username]<!-- auto name hack -->");

with this:

  $replacewords=array(0 => iif($bbuserinfo['usergroupid']==6,"$bbuserinfo[username]<!-- auto name hack -->",'{bbusername}'));