I couldn't find anything in your settings to account for that presently, so I'm not sure if you may have fixed this already? Creating a discount without anything in the Discount Code field would mean that discount gets applied automatically to every order which meets the Min. Item Quantity and Min. Item Total thresholds you define for the discount.
Also bear in mind that each buyer's cart persists for up to 24 hours or until they complete checkout, whichever happens first; i.e., if they'd entered a discount code in their cart yesterday and never checked out, then that discount would still be applied to their cart today.