If he only thing these invoices have in common is that they are for instance Accounts payable thaen it is possible that someone has created a rule to send all invoices to one user based only on the document type. You can't over rule these or create exceptions so you need to find out if you really want a rule that works like that.
You can see these rules by clicking the Rules icon (like a filled out sheet of paper on the left side of the Worklist). Rules open in their own tab. Choose the right client and see if you can find a rule that explains what is happening. You can edit the rule so it works only when you want it, or delete it.