For this to occur the same Vendor needs to be selected as the Preferred Vendor on the Item Record.
To check this:
Go to the Item Record> Purchasing/Inventory tab> Vendors subtab
- Ensure there is a Vendor selected here and the Preferred column is checked
All of the items that have the same Preferred Vendor set on the Item record will be added to the same Purchase Order that is created from the Sales Order link on the item level.