Controlling Invoice Holds

We have vendor invoices, that are matched to PO's, and automatically get put on hold.  This is good.  But when we change the pay group for one of these vendors, the hold is not generated.

Can anyone tell me what causes holds to be placed and how can it be controlled.  There are no hold settings set in the control tab of the supplier form, either on the supplier or site levels.

Thanks.

Bruce