Default commit option on sales order preference is set to "Do not Commit".
If this is your preference, all items will be back ordered upon creation of sales order regardless if you have enough to commit and assigned a location.
Please note that Items are not committed to this line item until this setting is changed.
To check/change preference:
a. Navigate to Setup > Accounting > Accounting Preferences.
b. Under Order Management tab > Sales Orders.
c. Default commit option on sales order = set to either:
- Available Qty - Items for this line item are committed as available. Available items are shipped, and items that are not available are placed on backorder.
- Complete Qty - This line item only ships when all items are committed.