This help page explains our discount settings in detail:
As explained there and on the discount settings screen itself, any discount with a blank Discount Code automatically gets applied to every order which meets the discount's Min. Item Qty./Total thresholds. Check your existing discounts to look for one without a Discount Code.
It's possible you may have inadvertently left that discount in place if you tried to change its Discount Name setting after you originally submitted it. Changing an existing Discount Name actually creates a new discount under the new Name without deleting the discount under the original Name, so if you didn't want to keep that, you'd need to deliberately delete the discount under the original Discount Name. Our new Admin panel currently in Beta testing (see the yellow bar at the top of our site) eliminates this technicality by making the Discount Name field read-only for existing discounts.