BOM item getting marked against purchase order when firming production orders

I apologize if this has been answered; I did search for it thoroughly first...

I'm using DAX 2009.  Sometimes when a planned production order is firmed to a scheduled work order, some of the BOM items get marked against an open purchase order.  Ultimately, these items get backflushed against the purchase order location rather than the work center location (which is what it should be).

We don't want any item to ever be marked.  I have looked everywhere I can think of for a setting that might cause this.

Any ideas or suggestions?

TIA