The Rate is changed if the Quantity in the Sales Order is edited even if Price Level has not been toggled

1.  Check if Set up > Company > Enable Features > Transactions, Quantity Pricing = True
2.  Check the item record, Lists > Accounting > Items > Pricing, is the price the same on the Sales Order?  Or has it been updated? 
 
When Quantity Pricing feature is turned on, and the quantity of an existing Sales Order has been updated, the system will double check if the rate is still within the coverage of the Quantity.  That being the case, the rate would be refreshed and updated to the current price.