Cancelled stock request rolls over to Purchase Order after PO Auto Sourcing runs

When a user puts in a requisition, PO Auto Source picks it up and sources it from inventory if we have it in stock. However, if someone at the warehouse cancels the stock request, it will sometimes cycle back to a purchase order. If the buyer cancels the PO without marking it to not resource, it will bounce back and forth between the warehouse and purchasing. Sometimes a canceled stock request becoming a purchase order is a good thing, but most of the time it is not. What causes this to occur?